We demonstrated immunohistochemically an abnormal expression of the neural cell adhesion molecule L1 in 10 developing brains of children with hemimegalencephaly (HM) aged from 36 weeks gestation to 10 years of age, comparing them with 23 controls aged from 13 weeks of gestation to 14 years. There was dense L1 expression in focal regions of the molecular layer beneath leptomeningeal glioneuronal heterotopia, in areas of cerebral cortex with large neurons, and in the disorganized or neuronal heterotopic sites in the white matter in HM. L1 was also heterogeneously enhanced in the abnormal cortex after 1 year of age, suggesting that axonal growth was delayed. These changes persisted into the older age group in the abnormal areas of cortex in HM. The cell bodies of many enlarged neurons in HM were immunopositive for L1, whereas L1 was usually localized to the processes of normal neurons. The delayed L1 immunoreactivity and enlarged L1-immunopositive neurons may be closely related to the pathogenesis of unilateral megalencephaly with cortical dysplasia and heterotopia.