BACKGROUND: The relationships of early growth with coronary heart disease and type 2 diabetes have received considerable attention. It is not known whether fetal or childhood growth is linked with autoimmune disorders. OBJECTIVE: Our objective was to assess whether the risk of adult-onset spontaneous hypothyroidism is predicted by body size at birth and during childhood. DESIGN AND SETTING: We conducted a birth cohort study in Helsinki, Finland. PARTICIPANTS: A total of 293 women who were born between 1934 and 1944 and had their heights and weights recorded at birth and during childhood participated in the study. MEASUREMENTS: We measured spontaneous hypothyroidism, defined as: 1) a disease history confirmed from medical records, or 2) previously undiagnosed hypothyroidism (TSH > 10 mU/liter). RESULTS: Twenty women (6.8%) had spontaneous hypothyroidism; 18 had been diagnosed previously, between 43 and 65 yr of age, and two had undiagnosed subclinical hypothyroidism. In addition, 59 women were thyroid peroxidase antibody positive. Compared with the 214 thyroid peroxidase antibody-negative women with no thyroid disorder, those with spontaneous hypothyroidism had on average 252 g [95% confidence interval (CI), 61 to 443 g; P = 0.01] lower birth weight and 1.2 cm (95% CI, 0.5 to 2.0 cm; P = 0.002) shorter length at birth. The odds of developing hypothyroidism increased 4.4-fold per kilogram decrease in birth weight (95% CI, 1.4 to 14.1). Hypothyroid subjects had been shorter in early childhood and had lower body mass index during later childhood. CONCLUSIONS: Small body size at birth and during childhood increases the risk of spontaneous hypothyroidism in adult women.