We and others have described previously a polymorphism at the first position of codon 52 (C52 --> A52) of the human thyrotropin receptor (hTSHr) gene. To determine its potential significance, we studied female (n = 100) and male (n = 25) patients with autoimmune thyroid disease (Graves' disease, n = 91; Hashimoto's thyroiditis, n = 34) and normal individuals [n = 121, female (n = 69), male (n = 52)]. Screening was performed using AciI restriction enzyme digestions of PCR-amplified genomic DNA. All codon 52 polymorphisms were verified by direct DNA sequencing. Data were analyzed using Chi-square or Fisher exact tests and p-values were corrected for multiple comparisons. Our studies demonstrated that this polymorphism is highly associated with autoimmune thyroid disease in the female population (corrected p = 0.008). We found no such association in the male population. Within females, there was a greater association between Graves' disease and the polymorphism (corrected p = 0.017) than between Hashimoto's thyroiditis and the polymorphism (corrected p = 0.090). The polymorphism was present in a higher proportion of Graves' disease patients with Graves' ophthalmopathy and pretibial dermopathy (40%) or Graves' ophthalmopathy, pretibial dermopathy, and acropachy (60%), than in patients with Graves' disease alone (15%), or Graves' disease and Graves' ophthalmopathy alone (17%). In conclusion, a polymorphism (C52 --> A52) of the hTSHr is associated with autoimmune thyroid disease in females.