Circulating CA 125 levels were studied in patients with gynecologic cancer and pelvic inflammatory disease, and in pregnant women. The CA 125 level was elevated (greater than 35 U/ml) in 69% (9/13) of patients with active ovarian cancer, in 32% (7/22) of patients with active cervical or endometrial cancer, in 24% (11/46) of pregnant women, and in 33% (10/30) of patients with acute pelvic inflammatory disease. Sixty-three other patients with nonmalignant gynecologic disorders, including 15 patients with ectopic pregnancy, had normal CA 125 levels. The occurrence of elevated CA 125 levels in patients with pelvic inflammatory disease can limit the use of the assay for diagnosis of cancer in young women. Gynecologic tumors may be associated with inflammatory reactions that may contribute to elevated CA 125 levels in some cancer patients.