To investigate the sensitivity of the CA 125 immunoradiometric assay for occult ovarian neoplasia, serum CA 125 levels were retrospectively determined “blind” in specimens collected from 105 women who subsequently developed ovarian neoplasia, and from 323 matched controls. The distribution of CA 125 levels was very different between the case and control populations (p = 0.0001) over the entire collection-to-diagnosis interval (range 1–143 months). Median CA 125 levels for all cases, and for those collected more than 24, 36 or even 60 months prior to diagnosis, were always 18 U/ml or greater, compared with a median of 10.9 U/ml for controls. Half of the cases collected within the 18 months preceding diagnosis had CA 125 levels of more than 30 U/ml and one-third had levels greater than 65 U/ml. About one-fourth of those collected prior to 60 months before diagnosis had levels above 30 U/ml. In contrast, approximately 7% and 0.9% of controls had levels in excess of 30 or 65 U/ml, respectively. Elevations occurred in cases eventually diagnosed with localized or advanced cancer, and with borderline or obviously malignant disease. These results provide an insight into the preclinical biology of ovarian neoplasia that may help in designing methods for early detection of this disease, and demonstrate the usefulness of the JANUS serum bank as a resource in evaluating serum tests.