A women's sexual behavior affects her risk of acquiring pelvic inflammatory disease, but the risks have not been well characterized. To study the association between pelvic inflammatory disease and sexual behavior, we analyzed data from a multicenter, case-control study involving 712 women hospitalized with an initial episode of pelvic inflammatory disease and 2719 hospitalized control women without a history of pelvic inflammatory disease. Study participants provided information on their frequency of intercourse, number of recent sexual partners, and previous history of gonorrhea. Logistic regression methods were used to adjust for confounding factors. Women who reported having four or more sexual partners were over three times more likely to be hospitalized for pelvic inflammatory disease (relative risk 3.4; 95% confidence interval 2.2-5.3) than were women who reported only one recent sexual partner. To a lesser extent, frequent sexual intercourse and history of gonorrhea also increased a woman's risk of pelvic inflammatory disease. Frequent intercourse was a strong risk factor for pelvic inflammatory disease among a subgroup of women who were at low risk for acquiring a sexually transmitted disease: Married women with one recent sexual partner with intercourse six or more times per week had a risk of pelvic inflammatory disease of 3.2 (1.4-7.2) compared with similar women having intercourse less than once per week. Frequent intercourse, which does not by itself increase the risk of acquiring a sexually transmitted disease, may increase a woman's risk of pelvic inflammatory disease.