Clinical data and magnetic resonance imaging scans from 10 patients with lissencephaly were reviewed. Although 6 of the 10 patients were less than 1 year old at the time of their most recent examination, those who were old enough for developmental assessment were, with a single exception, severely developmentally delayed. All patients had anatomical features compatible with impaired neuronal migration but normal myelination and, consequently, adequate glial migration. The severity of the lissencephaly did not seem to correlate with the severity of the clinical manifestations. One patient showed less delayed development than anticipated, perhaps because the most severely involved region of the brain was the prefrontal area.