186 infertile women underwent standard infertility investigations (including hysterosalpingography and, in 87 women, laparoscopy) and tests for the presence of antibody to Chlamydia trachomatis. 73% of the women with distal occlusion of the fallopian tubes and 21% with peritubal adhesions alone had antibodies to C. trachomatis, but none of those with normal tubes did (p less than 0.001 and less than 0.005, respectively). No other infertility factors were associated with an increased frequency of antibodies to C. trachomatis. Since the presence or absence of antibodies to C. trachomatis was as discriminatory in the detection of tubal disease in infertile women as was the hysterosalpingogram, the serological test for C. trachomatis should become part of a routine infertility investigation.