Purpose: Serum CA-125 level is commonly used as indicator for ovarian cancer recurrence. However, its value for the prediction of neoplastic lesions is unknown. The aim of this study was to investigate whether CA-125 concentrations are indicative of adnexal dysplasia and cancer in women at hereditary high risk of ovarian/tubal cancer. Patients and Methods: CA-125 was obtained from 424 women at hereditary high risk of ovarian/tubal cancer attending the VU University Medical Center (Amsterdam, the Netherlands) between 1993 and 2005. Serum samples obtained at the second-to-last (n = 64) and last (n = 98) visit before surgery were tested in women who underwent adnexal surgery for diagnostic (n = 9) or prophylactic (n = 89) reasons. Serum samples obtained from 370 age-matched healthy women were used as controls. Results: Both the absolute value (P < .0001) and the serial change (P < .0001) of CA-125 were predictive for ovarian cancer (n = 8). For adnexal dysplasia (n = 23), the absolute value of CA-125 (P = .003) was predictive, but the serial change in CA-125 was not (P = .32). The odds ratio for adnexal dysplasia versus nondysplasia in the highest tertile (CA-125 levels ≥ 14 U/mL) compared with the lowest tertile (CA-125 < 10 U/mL) was 6 (95% CI, 1.32 to 36.66). Conclusion: In patients at hereditary high risk for adnexal cancer, both the absolute value of serum CA-125 and the change in serial CA-125 are predictors for ovarian cancer. Remarkably, the absolute value of CA-125 is also predictive for adnexal dysplasia. CA-125 values should, therefore, be taken into account in the decision toward prophylactic bilateral salpingo-oophorectomy.