We studied 43 children with extensive brain gyral anomalies diagnosed radiologically and defined by (a) the absence or paucity of sulci over cortical areas affecting at least two lobes in each hemisphere, and (b) the absence or reduction of interdigitation between gray and white matter. We correlated the clinical, EEG, and genetic findings with the imaging features. A seemingly homogeneous group of patients (group A, n = 30) presented a common imaging pattern characterized by four features: (1) a thickened neocortex, (2) widened lateral ventricles, (3) apparent verticalization and widening of sylvian fissures, and (4) bilateral and symmetric distribution of the abnormalities. Another group of patients (group B, n = 13) exhibited heterogeneous imaging anomalies, termed "nonlissencephalic brain malformation," differing in at least one of the following four ways from the radiologic criteria defining group A: absence of verticalization of sylvian fissures (n = 12), thin neocortex (n = 2), normal-size lateral ventricles (n = 2), and asymmetric brain defects (n = 3). In group A, some clinical features had a significantly lower frequency (p < or = 0.01) than in group B: microcephaly, a complete lack of postural development, and intractable epilepsy. There was a significant relationship, but only in group A, between the degree of gyral anomalies and the extent of neurodevelopmental delay. Some EEG patterns (rapid rhythms and delta-theta rhythms) were highly specific for the group A patients. There was lower risk of familial recurrence in group A (recurrence of convolutional anomalies was 3.5% of sibship in group A versus 44% of sibship in group B, p = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)