Forty children with partial agenesis of the corpus callosum have been prospectively identified and followed up for up to 15 years. Additional physical anomalies and diagnoses were frequent. At the time of diagnosis, the majority of children were developmentally delayed or retarded. If callosal agenesis was diagnosed during infancy, most children had or subsequently developed seizures. Older patients had a more favorable outcome and were less likely to develop epilepsy.