The purpose of this report is to record two cases of agenesis of the corpus callosum in siblings and to discuss some of the possible causes of its occurrence. The literature on this anomaly was reviewed by Carpenter and Druckemiller in 1953.1They found 45 cases in which encephalograms had been obtained, and the diagnosis had been made during life in 43. They added two cases of their own. It is obvious that not all cases have been reported, and that therefore this anomaly occurs far more frequently than these figures would suggest. Their report does not include any instance of two cases occurring in the same family. CASE MATERIAL The patients were two sisters, Carol T. and Gail T. (Cases 1 and 2 respectively), admitted to the Montreal Neurological Institute on March 18, 1952, for investigation of mental retardation and physical underdevelopment. The father, aged 43, was well