Agenesis of the corpus callosum, both partial and complete, has been reported repeatedly as an unexpected autopsy observation. The five cases described in this communication are the first to be recognized during life, but the ventriculograms when once seen and understood could scarcely be mistaken for those showing any other condition. It is anticipated, therefore, that when clinicians generally recognize the ventriculographic features of these cases, the condition may be found to be less rare than is supposed and further knowledge of the function of the corpus callosum may be forthcoming. Up to the year 1933, Baker and Graves1could find only eighty-two cases of this condition reported in the medical literature, although the first example was described as long ago as 1812, by Reil.2A brief analysis of the normal and abnormal development of the corpus callosum and a discussion of published views on its function are