Macrophage colony-stimulating factor is a cytokine that stimulates proliferation and differentiation of phagocytic cells. Macrophage colony-stimulating factor is produced by ovarian epithelial cancer cell lines and might provide a useful serum marker for the disease. Among sera from 69 patients with clinically apparent epithelial ovarian cancer, 47 (68%) had at least 2.5 ng/ml macrophage colony-stimulating factor, whereas only two of 80 apparently healthy donors (2.5%) had a comparable elevation of macrophage colony-stimulating factor. Circulating levels of macrophage colony-stimulating factor did not correlate with serum levels of CA 125. Moreover, 14 of 25 ovarian cancer patients (56%) with clinically evident disease and normal levels of CA 125 (less than 35 U/ml) had elevated levels of macrophage colony-stimulating factor. Among 29 patients with serum CA 125 levels less than 35 U/ml before positive surgical surveillance procedures, 9 (31%) had at least 2.5 ng/ml macrophage colony-stimulating factor. Elevated levels of macrophage colony-stimulating factor were also found in patients with carcinomas from other primary sites and in 31% of 134 patients with benign diseases. If intercurrent benign disease can be taken into account, macrophage colony-stimulating factor deserves further evaluation in combination with CA 125 in monitoring ovarian cancer.