Although extensive studies have suggested that an individual's predisposition to develop autoantibodies to thyroid peroxidase (TPOAb) and thyroglobulin (TgAb) has a genetic component, a recent report has suggested that growth restriction during fetal life may also contribute. We have therefore measured TPOAb and TgAb in a population-based study of twins (44 monozygous and 91 dizygous) born between 1950 and 1955 who were identified through a registry of birth records in Birmingham, United Kingdom. TPOAb and TgAb were measured by a highly sensitive immunoprecipitation assay in which 0.01 U/mL and 0.1 U/mL, respectively, were detectable. TPOAb were found in 49 of 156 (31%) of women and 7 of 116 (6%) of men with similar values for TgAb (31% and 9%, respectively). Of 28 monozygous pairs, 10 were concordant for TgAb and 7 discordant giving a probandwise concordance rate of 74%. Concordance rates for TPOAb were similar (64%) and were higher than concordance rates in dizygous twins. Monozygous twins had a higher prevalence of TgAb than dizygous twins (30% vs. 17%, p = 0.01). In addition, we found that where there was discordance in birth size within monozygous twin pairs, the smaller twin had a higher prevalence of TPOAb (p = 0.01). Our results suggest that both heritable and early environmental components contribute to the susceptibility to thyroid autoimmunity.