The pathoetiology of attention-deficit hyperactivity disorder (ADHD) has been considered to be neurodevelopmental, yet the timing and processes involved are not clearly identified. Neurodevelopmental brain anomalies have been associated with a variety of psychiatric conditions. However, they have never been evaluated in a population of patients with ADHD. This study was designed to determine the frequency of specific developmental brain anomalies in a group of children with ADHD (n = 85; mean age, 10.9 years) and healthy control children (n = 95; mean age, 11.7 years) by visually inspecting brain magnetic resonance imaging scans. Compared to controls, the ADHD group showed an increase in frequency of two developmental anomalies: (1) gray-matter heterotopia, a neuronal migration anomaly, in 2 of 85 patients versus 0 of 95 controls; and (2) posterior fossa abnormality (excess cerebrospinal fluid in the posterior fossa) in 8 of 85 patients versus 2 of 95 controls. There were no differences in frequency of enlarged cavum septi pellucidi between the two groups. These findings support and extend the idea that ADHD is of developmental origin, and further suggest that the timing of aberrant brain development could be in early gestation.