We report the results of our investigation using DNA markers, of the parental origin of 61 XXY males ascertained during cytogenetic studies of consecutive liveborn babies and compare our results with those obtained from a study of 50 XXY males ascertained through clinical referral or amniocentesis. In the former group 44% of newborn XXY males obtained the additional X chromosome from their father and 56% from their mother, while in the latter "referral" group the additional X chromosome was paternally derived in 54% and maternally derived in 46% of the cases. The precise maternal cell division at which nondisjunction occurred was determined in 39 cases of maternal origin. Twenty eight (72%) resulted from an error in the first meiotic division and 11 (28%) from an error in the second meiotic division. There was no evidence of an origin due to a postzygotic mitotic error. There was no significant difference in the parental age at birth between those patients who received the additional X from their father and those who received it from their mother. However, those patients who received the additional X chromosome as a result of a maternal meiotic I error had a higher maternal age than any other category, but the difference reached a level of formal significance only for the clinically referred patients.