The birth prevalence of sex chromosome trisomies (SCT), that is individuals with an XYY, XXY or XXX sex chromosome constitution, is traditionally based on six surveys of unselected newborns carried out in the 1960s and early 1970s. All three SCTs had a prevalence of 1 in 1000 same sex births. We re-examined these prevalences based on additional cytogenetic studies of newborn surveys, spontaneous abortions, perinatal deaths and prenatal diagnoses. The more recent newborn surveys suggest there has been an increase in the prevalence of XXYs, but not of the other two SCTs since the original newborn series. The prevalence of XXYs has risen from 1.09 to 1.72 per 1000 male births (P=0.023). We suggest that such an increase, in the absence of an increase in the prevalence of XXX, is unlikely to be due to increased maternal age. As XXY is the only chromosome abnormality known where a substantial proportion ( approximately 50%) arise as the result of non-disjunction at the first paternal meiotic division, we speculate that some factor may be interfering with pairing and/or recombination of the sex bivalent at the paternal MI division.