Aims/hypothesis To describe the associations between age, sex and BMI at diagnosis of type 2 diabetes, and test the hypothesis that men are diagnosed with diabetes at lower average BMI than women of similar age. Methods Linear regression was used to estimate and compare the relationship between age and BMI at diagnosis among 51,920 men and 43,137 women included in a population-based diabetes register in Scotland for whom an index BMI measurement was taken within 1 year of diabetes diagnosis. We also examined HbA1c values by sex within the same timescale. Results Mean BMI closest to date of diagnosis of type 2 diabetes mellitus was 31.83 kg/m2 (SD 5.13) in men and 33.69 kg/m2 (SD 6.43) in women. The inverse relationship between age and BMI at diagnosis of type 2 diabetes mellitus was significantly steeper in women than in men (slope estimate in men −0.12 kg/m2 per year [95% CI −0.13, −0.12] women −0.18 kg/m2 per year [95% CI −0.18, −0.17], p < 0.0001 for formal test of interaction). Mean BMI difference was most marked at younger ages and narrowed with advancing age. However, HbA1c levels within 1 year of diagnoses were broadly similar in men and women. Conclusions/interpretation Men are diagnosed with type 2 diabetes at lower BMI than women across the age range. This observation may help explain why type 2 diabetes is more common among middle-aged men in populations of European extraction. Whether the same pattern is also observed in other ethnic groups requires confirmation.