Chromosome heteromorphisms of parents and their trisomic spontaneous abortions were compared in an attempt to determine the parental origin of 204 single trisomies, including cases of trisomy 3, 4, 9, 13, 14, 15, 16, 21 and 22, nine mosaic trisomies and nine double trisomies. Non-disjunction at maternal meiosis I was the most likely source of the additional chromosome for all trisomies studied, including the mosaics, and this was the case at all maternal ages. However, trisomy 21 had a significantly increased proportion of paternally derived cases by comparison with all other trisomies. Consideration of the sex ratio in cases of trisomy 21 of known parental origin suggests that there is an excess of males associated with paternal first meiotic division non-disjunction. The fact that this mechanism of origin is more prevalent in trisomy 21 may well explain why there is an excess of males associated with this abnormality but not with other autosomal trisomies.