Results of a cytologic study of 5 children, all males and white, with generalized congenital analgesia, belonging to 3 different families, are presented. In 4 of the patients as well as in the father of 2 of them, short-term cultures of peripheral blood showed the existence of abnormal karyotypes containing 1 or 2 extra chromosomes identified as belonging to the 13-15 group. Despite the quantitative variation of the frequency of abnormal cells, all hypermodal cells analyzed did present the same extra acrocentric. The patient in which the extra chromosome was not found might have been of the mosaic type too, but with trisomic cells in other tissues. The fact that patients presented the chromosome anomaly only in the form of cell mosaicism allows us to assume that presence of this relatively large extra chromosome in all cells of an individual would lead to lethality at an early stage.