To assess the use of CA 125 as a potential screening test for ovarian cancer, it is necessary to understand how the test performs among cancer-free women, who would constitute the overwhelming majority of those being tested at any time. Two hundred fifty-eight menopausal volunteers who were not seeking gynecologic care had CA 125 measured on two occasions and also underwent transvaginal ultrasonography to measure ovarian volumes. Only one subject had a CA 125 level greater than 35 U/mL. The mean value for CA 125 was 5.6 +/- 3.5 U/mL. Within women, the correlation between two tests was very high (r = 0.82). Among women with apparently normal ovaries, CA 125 values were low, tightly distributed, and reproducible; these findings indicate that the test may prove to be useful and cost-effective in a screening program. The correlation between CA 125 values and ovarian volume determined by sonogram was low (r = 0.11). The independence of these two tests would also be desirable in any screening program. Other studies have found normal women to have higher levels of CA 125 with wider distributions than found here; further evaluation of test performance at these low levels is needed.