Although pelvic inflammatory disease is the most common serious infection among young women of reproductive age in the United States, no nationwide data are available on the patterns of antibiotic treatment of this disease. To examine these patterns we analyzed over 25 million antibiotic prescriptions for treatment of pelvic inflammatory disease from 1966 through 1983, using the National Disease and Therapeutic Index. Most patients received a single antibiotic on an outpatient basis. Overall, use of natural penicillins declined markedly, and use of aminopenicillins more than doubled. Cephalosporins emerged as the most frequently prescribed antibiotic for hospitalized patients. Nationwide surveillance of treatment patterns may help to identify areas needing improvement through continuing education.