Lissencephaly (agyria-pachygyria) is the most severe neuronal migration disorder, characterized by total or partial absence of gyri. In this study, 21 patients with lissencephaly type I (9 girls, 12 boys) with a mean age of 19 +/- 21 months (2 weeks-8 years) were evaluated clinically and graded according to neuroradiological findings (19 patients by magnetic resonance imaging MRI and 2 by computed tomography CT). Three patients were classified as lissencephaly grade 2 and 18 patients as grade 3 or 4. Clinically, 12 patients (57%) had microcephaly, and eight (38%) had facial dysmorphism. All the patients had prominent psychomotor retardation, moderate to severe; the most frequent neurological findings were spastic guadriplegia (36.4%) and hypotonia with exaggerated tendon reflexes (27.3%). Seventy-eight percent of the patients had epileptic seizures resistant to conventional treatment. Lissencephaly is a cerebral cortical malformation that should be considered in children with developmental delay with or without microcephaly and facial dysmorphism. In addition, it should be investigated in the etiology of early-onset childhood epilepsy.