It is clear that when children with attention-deficit/hyperactivity disorder (ADHD) become adults, many continue to display manifestations of inattention, hyperactivity, and impulsivity (Barkley, Fischer, Smallish, & Fletcher, 2006; Mannuzza, Gittelman-Klein, Bessler, Malloy, & LaPadula, 1993; Weiss & Hechtman, 1993). As such, ADHD in adults is characterized by a range of impairments in daily life functioning (Barkley, Murphy, & Fischer, 2008). For example, compared with their peers, young adults who were diagnosed with ADHD in childhood are far less likely to pursue higher education, hold a steady job, responsibly manage their finances, and maintain adaptive social relationships (Barkley et al., 2006, 2008; Weiss & Hechtman (1993). These individuals also are more likely to experience dangerous problems with driving, risky sexual behavior, substance abuse, intimate partner violence, and criminal behavior (Barkley et al., 2008; Derefinko & Pelham, in press; Flory, Molina, Pelham, Gnagy, & Smith, 2006; Mannuzza, Klein, & Moulton, 2008; Thompson, Molina, Pelham, & Gnagy, 2007; Weiss & Hechtman, 1993; Wymbs et al., in press). Despite these poor outcomes, ADHD in adulthood is poorly defined and somewhat controversial (Barkley, 2006). A major reason for this confusion is that the field lacks clear evidence-based methods for identifying ADHD in adulthood. Studies applying strictly interpretation of Diagnostic and Statistical Manuel of Mental Disorders (DSM; American Psychiatric Association, 1980–2000)) diagnostic criteria report low to moderate persistence rates (4%-42%; Barkley, Fischer, Smallish, & Fletcher, 2002; Kessler et al., 2005; Mannuzza, Klein, Bessler, Malloy, & LaPadula, 1998; Mannuzza, Klein, & Moulton, 2003). Therefore, experts initially believed that ADHD tends to remit after adolescence (Barkley et al., 2002; Mannuzza et al., 1998). However, other studies report higher persistence rates (49%–66%) by defining diagnostic threshold according to the presence of significant impairment or elevated symptomatology (as compared with control-group norms; Barkley et al., 2002; Weiss & Hechtman, 1993). These data suggest that, as adults, individuals with childhood ADHD display the core symptoms of the disorder and serious dysfunction (Biederman, Mick, & Faraone, 2000) but meet criteria for fewer of the DSM ADHD symptoms than children. Recent work on ADHD in adults aims to better characterize its symptom expression. However, the typical adult with ADHD may not be well represented in these samples. Namely, most diagnostic research involves self-identified clinical samples of adults with ADHD (Barkley et al., 2008; Biederman et al., 2006). Typically, up to 50% of adults in these samples are women (e.g., Biederman et al., 2006), and sample participants often are not required to possess a childhood history of significant ADHD-related impairment (e.g., Barkley et al., 2008), making their composition different from samples identified in childhood, who possess a standard ADHD diagnosis. Namely, adults with a lifetime history of ADHD tend to underestimate their problems (Barkley et al., 2002), rarely present for ADHD treatment in adulthood, and therefore are unlikely to be included in adult-referred clinical samples. As a result, confusion over the expression of ADHD in adulthood may stem from research with somewhat unrepresentative samples. Beyond “who” is included in the aforementioned samples, correct characterization of ADHD in adulthood hinges upon “how” information about these individuals is obtained. As mentioned previously, the typical adult with a childhood history of ADHD tends to dramatically underreport his or her own problems (Barkley et al., 2002; Sibley et al., 2010). Yet, most research with adult-diagnosed samples relies solely on self-report, which only appears valid for these self-referred individuals (Barkley, Knouse, & Murphy, 2011). Specifically, reports from informants such as parents (Barkley et al., 2002), siblings (Loney, Ledolter, Kramer, & Volpe, 2007), and other adults (Barkley et al., 2008) appear to offer more valid ratings of adults with an established childhood history of ADHD. Furthermore, most adult-diagnosed samples either do not require the DSM “B” criterion be met (ADHD symptoms in childhood; Barkley et al., 2008; Biederman et al., 2006) or rely solely on retrospective self-report to assess childhood functioning (Faraone et al., 2006; Kessler et al., 2010). Research is mixed with regard to the ability of adults with ADHD to provide accurate retrospective report of their childhood functioning (Mannuzza, Klein, Klein, Bessler, & Shrout, 2002; Miller, Newcorn, & Halpern, 2010). Therefore, further work is needed in this area. The recent studies with adult-diagnosed samples universally suggest that the DSM needs new developmentally appropriate items for ADHD in adulthood (Barkley et al., 2008; Faraone, Biederman, & Spencer, 2010; Kessler et al., 2010). Undoubtedly, some of the DSM–IV (DSM–4th ed., American Psychiatric Association, 1994) symptoms are inappropriate descriptors of adults (e.g., difficulty playing quietly, inappropriate running and/or climbing). A string of studies asserts that combining adult-specific items with several developmentally ubiquitous ones (e.g., easily distracted, difficulty organizing tasks, difficulty sustaining attention) creates an adult-ADHD algorithm that improves upon the DSM's diagnostic utility (Barkley et al., 2008; Faraone et al., 2010; Fedele, Hartung, Canu, & Wilkowski, 2010; Kessler et al., 2010). However, these findings are limited by factors discussed previously, namely, the use of adult-referred samples and self-report information. Consequently, there are now several recommended sets of adult-specific ADHD items that possess very little overlap with each other (Barkley et al., 2008; Faraone et al., 2010; Kessler et al., 2010; Wender, 1985). To date, adult-specific ADHD items have not been examined using a sample of adults with established ADHD in childhood that provides both self- and informant-report of functioning. Using these methods might elucidate the expression of ADHD in adulthood and the utility of adult-specific items. In sum, further work is needed to understand and standardize the diagnosis of ADHD in adulthood. Thus, in the current study, we aimed to develop recommendations for an adult-ADHD diagnostic protocol by examining the symptoms and functioning of young adults in the Pittsburgh ADHD Longitudinal Study (PALS; Molina, Pelham, Gnagy, Thompson, & Marshal, 2007). The PALS includes a sample of young adults who were well diagnosed with ADHD in childhood using standard DSM criteria applied in a specialty clinic setting. We first compared estimates of ADHD persistence into young adulthood by examining rates of DSM–IV–TR (4th ed., text rev.; American Psychiatric Association, 2000) diagnosis (A criteria), elevated ADHD symptomatology, and clinically significant functional impairment. We hypothesized that a majority of the sample would continue to display elevated ADHD symptomatology and clinically significant functional impairment in young adulthood, but that fewer would meet DSM criteria for ADHD. With regard to the utility of informant report, we hypothesized that young adults with ADHD would underreport their current and childhood symptomatology and that parent report alone would be the most useful method of assessing ADHD. Next, we evaluated the performance of adult-specific item sets posited by several research teams (Barkley et al., 2008; Faraone et al., 2010; Kessler et al., 2010) relative to item sets based on DSM-IV criteria. To do so, we compared symptom endorsement rates, parent- and self-report agreement, and convergent validity for each of these item sets within the PALS ADHD and non-ADHD control groups. We hypothesized that across these indices, the adult-specific items would possess greater diagnostic utility than the DSM items.