The prevalence of humoural IgG and IgM antibodies to Chlamydia trachomatis was determined in 110 infertile women and compared to 87 healthy pregnant women without any known fertility problem. Overall antibodies to chlamydia were detected in 45% of infertile women. Antibodies were found in significantly more patients with tubal factor infertility (65%) than in women whose infertility was due to other causes (22%) (p less than 0.005). These findings are consistent with the hypothesis that C. trachomatis is a major cause of tubal factor infertility. In addition the prevalence of antibody in patients with other sexually transmitted diseases (STD), pelvic inflammatory disease and confirmed chlamydia cases were evaluated. Within this miscellaneous high risk group of patients, chlamydial antibodies were detected commonly, ranging from 19-72%.