Low birth weight is a well-established risk factor for type 2 diabetes, but the risk at high birth weight levels remains uncertain. Potential sex differences in the associations are unexplored. We investigated whether sex influences the association of birth weight and adult type 2 diabetes, using a cohort of 113,801 men and 109,298 women, born 1936-1983, from the Copenhagen School Health Records Register, Denmark. During 5.6 million person-years of follow-up, 7,750 men and 4,736 women had a diagnosis of adult type 2 diabetes (30 years of age or older) obtained from national registers. When birth weights between 3.251 and 3.750 kg were used as the reference group for each sex separately, women with birth weights in the categories of 2.000 to 2.750 kg and 4.751 to 5.500 kg had hazard ratios [HRs] of type 2 diabetes of 1.46 (95% CI, 1.34-1.59) and 1.56 (1.20-2.04), respectively, whereas men had HRs of 1.20 (1.12-1.30) and 0.93 (0.76-1.15). Thus, sex modified the association, with stronger risk estimates of type 2 diabetes in women at both low and high birth weights compared with men (P = 0.001). In conclusion, birth weight is more strongly associated with type 2 diabetes in women than in men. Future search for sex-specific causal mechanisms may provide new insights into the early origins of type 2 diabetes.