Agenesis of the corpus callosum (ACC) is an infrequent congenital abnormality that has been diagnosed by necropsy, surgery, pneumoencephalography, computerized tomography (CT), and magnetic resonance imaging. The reported prevalence has varied as a function of disability status of the population studied and diagnostic technique. This report found 33 cases of ACC in a consecutive series of 1,447 CTs of developmentally disabled individuals. The prevalence, 2.3%, is consistent with studies using other techniques. However, the significantly higher (p less than 0.01) rate from 1978 to 1979 suggests an initial tendency of neuroradiologists to over-diagnose ACC, and CT data from the 1970s may need to be reexamined.