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Objectives Despite early and continuous treatment many patients with phenylketonuria (PKU) still experience neurocognitive problems. Most problems have been observed in the domain of executive functioning (EF). For regular monitoring of EF, the use of the Behavior Rating Inventory of Executive Function (BRIEF) has been proposed. The aim of this study was to investigate whether the BRIEF is indeed a useful screening instrument in monitoring of adults with PKU. Study design Adult PKU patients (n = 55; mean age 28.3 ± 6.2 years) filled out the BRIEF-A (higher scores = poorer EF) and performed computerized tasks measuring executive functions (inhibition, cognitive flexibility, and working memory). The outcome of the BRIEF-A questionnaire was compared with the neurocognitive outcome as measured by three tasks from the Amsterdam Neuropsychological Tasks (ANT). Results Forty-two percent of the PKU patients scored in the borderline/clinical range of the BRIEF-A. Six of the 55 patients (11%) scored > 1 SD above the normative mean, mostly on the Metacognition Index. With respect to ANT measurements, patients mainly showed deficits in inhibitory control (34–36%) and cognitive flexibility (31–40%) as compared to the general Dutch population. No significant correlations between the two methods were found, which was confirmed with the Bland–Altman approach where no agreement between the two methods was observed. Only with respect to inhibitory control, patients scored significantly worse on both BRIEF-A and ANT classifications. No other associations between classification according to the BRIEF-A and classifications according to the ANT tasks were found. Conclusions Patients reporting EF problems in daily life are not necessarily those that present with core EF deficits. The results of this study suggest that regular self-administration of the BRIEF-A is not a sufficient way to monitor EF in adult PKU patients. [ABSTRACT FROM AUTHOR]