In a case-control study of matched pairs, the risk of acute pelvic inflammatory disease (PID) was 4.4 times higher in intrauterine contraceptive device (IUD) users than in nonusers (p less than 0.001). Of approximately 500,000 cases of acute PID occurring annually in the United States, an estimated 110,000 are attributable to IUD's, costing over forty-four million dollars per year. PID was attributable to the IUD in 77 per cent of IUD users. No particular type of IUD was implicated. The relative risk of acute PID in IUD users over nonusers was higher in nulligravid women than in previously pregnant women and was directly related to socioeconomic status (SES), but the total annual risk of PID in IUD users appear inversely related to SES. IUD use significantly increased the risk of nongonococcal PID. Fever occurred in 13 (21 per cent) of 61 IUD users and 59 (41 per cent) of 143 nonusers (p less than 0.025). Among women with nongonococcal PID, and adnexal mass greater than or equal to 6 cm. was noted in 14 (40 per cent) of 35 IUD users and in only 12 (15 per cent) of 78 nonusers (p less than 0.01). An increased risk of gonococcal PID was found among non-Caucasians and women not using contraception, while the risk of nongonococcal PID was increased among women with a past history of gonorrhea. Oral contraceptive use may protect women with gonorrhea from developing PID. Menstruation precipitates the onset of symptoms of gonococcal PID.