Serum CA 125 levels were determined in 64 women with benign ovarian lesions, 92 women with uterine fundal lesions, and six patients who had negative second-look laparotomy for epithelial ovarian carcinoma. Of those with benign lesions, 13 of 31 patients with endometriosis had levels greater than 35 U/ml. Six of 34 patients with endometrial carcinoma had elevated levels before the primary operation, and six of 15 patients with recurrent endometrial carcinoma had elevated levels. The six ovarian cancer patients had had negative findings at second look 7 to 40 months before recurrence. Where close serial levels were available, the level became elevated 2 to 5 months before clinically apparent recurrent disease was noted.