We used X-linked DNA polymorphisms to study the parental origin of the missing or additional X chromosome in conceptuses with 45,X or 47,XXX chromosome constitutions. Fifty-three (80.3%) of 66 cases of sex chromosome monosomy had a maternal X, demonstrating that paternal sex chromosome loss is the most common error leading to this condition. In contrast, 29 (93.5%) of 31 47,XXX cases resulted from maternal nondisjunction. In studies of parental age, we detected no obvious parental age effect for sex chromosome monosomy. For the 47,XXXs, there was an apparent association between increasing maternal age and errors at maternal meiosis I, but not maternal meiosis II. In other analyses, we detected no obvious effect of parental origin on the phenotype of either 45,X or 47,XXX conceptuses, at least as measured by comparing the ratio of paternal:maternal errors among different ascertainment categories.