It is often assumed that there is little or no sex bias within either Type I (insulin-dependent) or Type II (non-insulin-dependent) diabetes mellitus. This review considers evidence that sex effects of interest and importance are present in both forms of the disease. Type I diabetes is the only major organ-specific autoimmune disorder not to show a strong female bias. The overall sex ratio is roughly equal in children diagnosed under the age of 15 but while populations with the highest incidence all show male excess, the lowest risk populations studied, mostly of non-European origin, characteristically show a female bias. In contrast, male excess is a consistent finding in populations of European origin aged 15–40 years, with an approximate 3:2 male:female ratio. This ratio has remained constant in young adults over two or three generations in some populations. Further, fathers with Type I diabetes are more likely than affected mothers to transmit the condition to their offspring. Women of childbearing age are therefore less likely to develop Type I diabetes, and – should this occur – are less likely to transmit it to their offspring. Type II diabetes showed a pronounced female excess in the first half of the last century but is now equally prevalent among men and women in most populations, with some evidence of male preponderance in early middle age. Men seem more susceptible than women to the consequences of indolence and obesity, possibly due to differences in insulin sensitivity and regional fat deposition. Women are, however, more likely to transmit Type II diabetes to their offspring. Understanding these experiments of nature might suggest ways of influencing the early course of both forms of the disease. [Diabetologia (2001) 44: 3–15]