The authors used enzyme immunoassay to determine the prevalence of serum antibodies to the sexually transmitted disease (STD) organisms Chlamydia trachomatis, Neisseria gonorrhoeae, and Mycoplasma hominis among 104 infertile women undergoing in vitro fertilization. Altogether, 55 (72%) out of 76 women with tubal abnormalities tested positive for one or more STD organisms, compared with only 6 (21%) out of 28 infertile women with normal tubes (P less than .001). The authors obtained positive test results for C. trachomatis, N. gonorrhoeae, and M. hominis in 40%, 14%, and 37% of the patients with tubal abnormalities, respectively; of women without tubal abnormalities, the test results were 7%, 0%, and 14%, respectively. Out of 20 patients with a history of ectopic pregnancy, the authors obtained positive findings for C. trachomatis, N. gonorrhoeae, and M. hominis in 8 (40%), 1 (5%), and 7 (35%), respectively. These results indicate an independent role for all three STD organisms in the etiology of tubal factor infertility and ectopic pregnancy following both symptomatic and asymptomatic pelvic inflammatory disease (PID). The correlation between positive mycoplasmal serology and secondary infertility and tubal abnormalities may suggest a link between M. hominis infections during pregnancy and delivery complications and consequent development of tubal factor infertility.