The increasing incidence rate of thyroid cancer warrants investigation of potentially modifiable factors, especially overweightness. Few prospective studies have investigated anthropometry from childhood to adulthood in relation to thyroid cancer. We analyzed data from 91,909 women of the E3N study, a cohort of French women insured by a national health scheme mostly covering teachers with the age of 40-65 years at inclusion in 1990. Risk estimates of first primary differentiated thyroid cancer (n = 317) were computed using Cox proportional hazards models. There was a significant dose-effect relationship between thyroid cancer risk and weight or body mass index (BMI) but not height. Compared with women whose BMI was 18.5-22 kg/m(2), women with BMI 22-25 and those with BMI over 30 had a 39% [95% confidence interval (CI) 7-81] and 76% (12-176) higher risk of thyroid cancer, respectively, with a 21% (5-39) increased risk per 5 kg/m(2) increase in BMI. A large body shape from age 35-40 was significantly associated with an increased risk of differentiated thyroid cancer when compared with a lean one, while earlier body shapes were not. The highest risk was observed in women whose body shape increased from lean to large between menarche and adulthood, with a HR of 2.17 (95% CI 1.04-4.53) when compared with women who were lean at both periods. Our study adds to the existing evidence in relation to excess weight to the risk of thyroid cancer, particularly in women whose body shape increased from menarche to adult age.