Chromosomal conditions were examined in 300 cases of various congenital diseases. The diseases were tentatively classified into 5 categories for convenience of description: Down''s syndrome, Klinefelter''s syndrome, Turner''s syndrome, hermaphroditism, and malformations. Chromosomes from a number of individuals were examined and are reported in great detail. Generally, abnormal chromosome complements in man are characterized by the involvement of sex chromosomes or the autosomes, but seldom of both in the same individual.