Elevated maternal thyroid autoantibodies during pregnancy are linked to infertility, miscarriage, and neurodevelopmental deficits such as in cognitive function. It has not been established whether autoantibodies to thyroid peroxidase are associated with sensorineural hearing loss (SNHL). The authors tested stored third-trimester maternal serum specimens of 1,736 children for thyroid peroxidase autoantibodies (TPOaAb) by using an enzyme-linked immunosorbent assay technique. The children participated at the Baltimore, Maryland, site of the Collaborative Perinatal Project, which enrolled pregnant women in 1959-1965. An audiology examination was administered to the children at 8 years of age and was used to identify cases of SNHL. Compared with 4.3% of the other children, 22.7% of the children whose mothers had elevated TPOaAb (> or =62.5 IU/ml) had SNHL. The difference was significant after controlling for maternal race, age, and hypothyroidism (exact prevalence odds ratio = 7.5, 95% confidence interval: 2.4, 23.3). When a lower cutoff of TPOaAb > or =31.25 IU/ml was used, there continued to be an association with SNHL (exact prevalence odds ratio = 5.7, 95% confidence interval: 2.1, 15.6). The direction and magnitude of the association were similar when an alternative case definition of SNHL was used. These data suggest that antenatal exposure to maternal TPOaAb during the third trimester of pregnancy is associated with impaired auditory development.