Background: Nonoccupational postexposure prophylaxis (nPEP) is recommended after a sexual or parenteral exposure to human immunodeficiency virus (HIV). Patients frequently seek care in an emergency department (ED) after an exposure and are usually referred to an HIV clinic for further management. There have been few data on determinants of attrition after presentation to EDs for nPEP., Methods: From July 2010 to June 2011, we prospectively recorded all referrals to nPEP programs from 2 large EDs at 2 academic medical centers in Boston, Massachusetts. Data were recorded on patient demographics, nature of potential HIV exposures, referrals to and attendance at HIV clinics, and reported completion of 28 days of antiretroviral therapy (ART). Multivariable logistic regression was used to evaluate risk factors for (1) patient attrition between the ED and HIV clinic follow-up and (2) documented completion of ART., Results: Of 180 individuals who were referred to clinic follow-up for nPEP care from the ED, 98 (54.4%) attended a first nPEP clinic visit and 43 (23.9%) had documented completion of a 28-day course of ART. Multivariable analysis revealed older age (adjusted odds ratio [aOR], 0.96; 95% confidence interval [CI], .93-.99) and self-payment (aOR, 0.32; 95% CI, .11-.97) were significant predictors for failing to attend an initial HIV clinic appointment. Women were less likely than men to complete a 28-day ART regimen (aOR, 0.34; 95% CI, .15-.79)., Conclusions: Commonly used nPEP delivery models may not be effective for all patients who present with nonoccupational exposures to HIV. Interventions are needed to improve rates of follow-up and completion of nPEP to reduce the risk of preventable HIV infections.