Objectives. To establish the gender difference amongst newly diagnosed type 1 diabetic patients aged 15-34 years, considering age at diagnosis, temporal trend and seasonal variation at time of diagnosis. Study design. A population-based prospective study with a mean annual population at risk of 2.3 million. Setting. All departments of medicine, endocrinology and paediatrics and primary health care units in Sweden. Subjects. Incident cases of diabetes aged 15-34 years at diagnosis 1983-2002. Measure instrument. Basic characteristics of patients at diagnosis were reported by the diagnosing doctor on a standardized form. Level of ascertainment was estimated at 80-90%. Results. Amongst all incident cases (n = 8012), 74% was diagnosed with type 1 diabetes. The mean annual incidence rate of type 1 diabetes was 12.7/100 000, in men 16.4/100 000 and in women 8.9/100 000. The incidence of type 1 diabetes decreased slowly by increasing age but was in all age groups higher in men, yielding an overall male/female ratio of 1.8. In both genders the incidence of type 1 diabetes decreased in average of 1.0% per year. A seasonal pattern with significantly higher incidence during January-March and lower during May-July was seen in both genders. Conclusions. A clear male predominance of type 1 diabetes was seen in all ages. The temporal trend and the seasonal pattern was similar in men and women. Hence, internal factors related to the gender rather than differences in the exposure to environmental factors seem to explain the consistent male-female bias in the postpubertal risk of developing type 1 diabetes. © 2008 Blackwell Publishing Ltd.