Serum samples were collected from 915 nonhospitalized Roman Catholic nuns with a median age of 55 years (range 19-94 years). Using an immunoradiometric assay, serum CA 125 levels ranged from 0-574 U/mL with a median of 10.5 and mean of 14.3 U/mL. Thirty-six women (3.9%) had serum levels greater than 35 U/mL, and only seven (0.76%) had serum CA 125 levels above 65 U/mL. In only 14 (2.4%) of 586 women aged 50 or older were CA 125 levels above 35 U/mL, and in only three (0.51%) of this group did levels exceed 65 U/mL. Among the seven women with levels above 65 U/ML, five were found to have benign or malignant neoplasms or other masses at the time of entry into the study or during the follow-up interval (mean 311 +/- 103 days). Moreover, in six of seven members of this "false positive" group, some disorder was diagnosed during the study period that might have elevated the CA 125 level. No correlation was found between serum CA 125 levels and a variety of nonmalignant disorders or a variety of concurrent medications. The apparent specificity of the CA 125 assay in this study population suggests that, if used in conjunction with other tests to discriminate ovarian carcinoma from disorders that could elevate serum CA 125 levels, this assay might be a potential component of a strategy aimed at the early detection of ovarian cancer.