An 11-month-old boy is described with severe developmental retardation, colobomata of both eyes, peculiar facies, syndactylies of the fingers and multiple minor malformations. Lymphocyte cultures showed the karyotype 48, XXYY, whereas, a skin fibroblast culture gave a mixture of diploid and triploid cells with the karyotypes 48, XXYY and 71,XXXYY respectively. In the triploid cell line only one of the three X chromosomes is allocyclic, as demonstrated by H3-TdR labeling and by sex chromatin studies.