364 women were studied for the possible role of chlamydia trachomatis infection as a factor related to infertility. C. trachomatis was isolated from the cervix in 20% of our high-risk STD patients and in a much lower proportion in patients with tubal (5%) or unexplained (4%) infertility. IgG-antibodies to C. trachomatis were found in 75% of patients with tubal infertility, in 44% of women without gynaecological diseases and in 87% of the high-risk STD patients. By contrast, the difference was significant in the IgA-antibody tests. 67% of the women with tubal infertility and only 25% of the women with nontubal infertility were positive. From these data it appears, that antecedent infections with C. trachomatis, as measured by antibody prevalence, are an important factor in infertility of tubal origin.