We studied selected epidemiologic, clinical, serologic, and microbiologic findings and their interrelationships among 57 women with acute pelvic inflammatory disease (PID). Cervical cultures positive for Neisseria gonorrhoeae alone and for both N. gonorrhoeae and Chlamydia trachomatis were associated with young age, nulliparity, and use of birth-control pills. Positive serologic findings for C. trachomatis were associated with the isolation of C. trachomatis and/or N. gonorrhoeae from the cervix and predicted the presence of a pelvic mass. High levels of antibody to Mycoplasma hominis were associated with increasing age and parity, and predicted a low concentration of C-reactive protein (CRP), a long hospital stay, and a high convalescent-phase erythrocyte sedimentation rate (ESR). Women with recurrent PID had higher titers of antibody to C. trachomatis than those with primary PID. The use of an intrauterine contraceptive device predicted high CRP, high acute-phase ESR, long hospital stay, and was frequently associated with positive serologic tests for M. hominis. These results demonstrate that the clinical picture of PID depends not only on the microorganisms involved but also on many epidemiologic factors such as age, contraceptive method, and parity.