OBJECTIVE To determine how childhood overweight, in conjunction with other life course weight characteristics, relates to the development of type 2 diabetes in adulthood. RESEARCH DESIGN AND METHODS Among 109,172 women in the Nurses' Health Study II, body fatness at ages 5, 10, and 20 years was assessed by recall using 9-level pictorial diagrams (somatotypes) representing extreme thinness (category 1) to obesity (category 9). Recalled weights at age 18 years and adulthood were used to derive BMI. Self-reported cases of type 2 diabetes were confirmed by supplementary questionnaire. RESULTS Somatotypes at ages 5 and 10 years were positively associated with diabetes risk (Ptrend < 0.0001). The adjusted relative risk (RR) of women with somatotype ≥6 (vs. 2) at age 5 years was 2.19 (95% CI 1.79–2.67) and at age 10 years was 2.57 (2.20–3.01). Increases in size by somatotype or by weight gain since age 18 were associated with increased risk. Compared with women who were never overweight at any age, women who were overweight as an adult (BMI >25 kg/m2) but not previously had an adjusted RR of 8.23 (7.41–9.15). The adjusted RR was 15.10 (13.21–17.26) for women who were also overweight at age 10 (somatotype ≥5) and 18 (BMI >25 kg/m2). Increased childhood size was not associated with risk among women who did not continue to be overweight in adulthood. CONCLUSIONS Increased body size starting from childhood is associated with a greater risk of diabetes in adulthood. However, women who become lean in adulthood do not have an increased risk.