To determine whether fetal and infant growth could influence susceptibility to autoimmune disease in adults, the occurrence of thyroid autoantibodies and autoimmune thyroiditis was studied in 305 women, aged 60-71, born in Hertfordshire and for whom details of birthweight, infant growth, and feeding were routinely recorded. Thyroglobulin autoantibody was detected in 37% of the women, thyroid peroxidase autoantibody in 41%, and autoimmune thyroiditis, defined as biochemical or clinical hypothyroidism in association with thyroid autoantibodies, in 5.6%. The proportion of women with thyroglobulin and thyroid peroxidase autoantibodies fell with increasing birthweight but was not related to weight at 1 year of age or the method of infant feeding. The prevalence of both autoantibodies rose with increasing adult body mass index but fell as the waist to hip ratio increased. These results demonstrate the importance of early environment in determining the susceptibility to autoimmune thyroid disease. The contrasting effects of adult body mass index and waist to hip ratio on antibody prevalence could be explained by their associations with different hormonal environments.