The risk of developing diabetes is higher in offspring of fathers than of mothers with insulin-dependent diabetes mellitus (IDDM). The reasons for this sex differential are unclear, as early studies were often selected and relatively small. We conducted a prospective study on the risk of IDDM in a cohort of 9,453 offspring from 5,255 Finnish parents with diabetes diagnosed before age 30 years. Age of first admission to the hospital was considered to be the age of diagnosis of IDDM in the offspring; IDDM occurred in 248 offspring. The risk of IDDM tended to be lower in the offspring of the same gender as the diabetic parent (adjusted risk ratio (RR) 0.78; p-0.50). When offspring were of same gender as the diabetic parent, male offspring had a higher risk of IDDM than female offspring (RR 2.28; 95 % confidence interval 1.53-3.38), whereas if the gender of the diabetic parent and the offspring were different, the risk in male offspring was lower (RR 0.43; 95 % confi-Key words Insulin-dependent diabetes mellitus, epidemiology, cumulative risk, sex differential, family history. Although it is known that genetic factors play a maj or role in the aetiology of insulin-dependent diabetes mellitus (IDDM), not all genetically predisposed individuals develop the disease. Only about 25 % of monozygotic twins are concordant for IDDM [1, 2] and only 15 % of all HLA identical siblings of a dia