Taller stature and obesity in adulthood have been consistently associated with an increased risk of thyroid cancer, but few studies have investigated the role of childhood body size. Using data from a large prospective cohort, we examined associations for height and body mass index (BMI) at ages 7 to 13 years with risk of thyroid cancer in later life. The study population included 321,085 children from the Copenhagen School Health Records Register, born between 1930 and 1989 in Copenhagen, Denmark, with measurements of height and weight from 7 to 13 years of age. These data were linked with the Danish Cancer Registry to identify incident thyroid cancer cases (1968-2010). Hazard ratios (HR) and 95% confidence intervals (CI) were calculated for age- and sex-specific height and BMI SD scores (SDS) using proportional hazards models stratified by birth cohort and sex. During follow-up (median = 38.6 years), 171 women and 64 men were diagnosed with thyroid cancer. Both height and BMI were positively associated with thyroid cancer risk, and these associations were similar by age at measurement. Using age 10 as an example, HRs per 1 unit increase in SDS for height (~6-7 cm) and BMI (~1.5-2 kg/m(2)) were 1.22 (95% CI, 1.07-1.40) and 1.15 (95% CI, 1.00-1.34), respectively. These results, together with the relatively young ages at which thyroid cancers are diagnosed compared with other malignancies, suggest a potential link between early-life factors related to growth and body weight and thyroid carcinogenesis.