BACKGROUND: The main objective of the present study was to assess the prevalence rates of attention deficit hyperactivity disorder, oppositional defiant disorder, conduct disorder, and features of autism spectrum disorders in trigonocephalic patients, using validated instruments and by ruling out the confounding influence of IQ. The second aim was to assess the association between extracranial anomalies and cognitive and/or behavioral problems in patients with trigonocephaly. METHODS: Objectives were studied in 82 trigonocephalic patients aged 4 to 18 years at the Erasmus Medical Center in Rotterdam, The Netherlands. Features of autism spectrum disorders were assessed using the Social Communication Questionnaire. Attention deficit hyperactivity disorder, oppositional defiant disorder, and conduct disorder were assessed with the Diagnostic Interview Schedule for Children-Parent Version. The presence and nature of extracranial anomalies were ascertained by a clinician. RESULTS: Mental retardation (IQ <70) was present in 9 percent of patients with trigonocephaly. Findings indicated a 70 percent versus 24 percent prevalence of psychopathology (attention deficit hyperactivity disorder, oppositional defiant disorder, conduct disorder, or features of autism spectrum disorder) in patients with IQ levels of, respectively, <85 and ≥85. In the latter group, psychopathology was not significantly more common than expected based on prevalence rates reported in community samples. Extracranial anomalies were significantly correlated with lower IQ levels. However, when adjusted for IQ, the presence of extracranial malformations was not associated with an increased risk of behavioral problems. CONCLUSION: The relatively high prevalence of behavioral problems in patients with trigonocephaly seems to be mainly attributable to the co-occurrence of trigonocephaly and low intelligence. CLINICAL QUESTION/LEVEL OF EVIDENCE: Risk, II. Copyright © 2012 by the American Society of Plastic Surgeons.