Serum Ca-125 levels were measured in 154 patients undergoing laparoscopy or laparotomy for infertility and/or pelvic pain. Pelvic endometriosis at various stages was observed in 102 of the patients, and 52 had a visually normal pelvis and served as a control group. We found a significantly higher mean serum Ca-125 concentration in the patients with stages III and IV endometriosis compared with the controls (P less than 0.001) whereas in the patients with stages I and II disease the mean antigen level was comparable to that of the control group. Considering its low sensitivity (14.7%) the usefulness of serum Ca-125 measurements as an initial diagnostic test for endometriosis is scanty. On the contrary, because of its elevated specificity (100%), this test may be useful in indicating early surgical exploration of the pelvis in cases of infertility and/or dysmenorrhea which are associated with high values of Ca-125.