The usefulness of serum CA 125 measurements in monitoring ovarian cancer has been firmly established in recent years. The CA 125 levels correlate well with tumor status, have a predictive value for second-look operations, indicate responsiveness to chemotherapy, and are a strong independent prognostic factor for survival. We report here a patient with an unusual evolution of the CA 125. The primary treatment consisted of surgery and whole-abdomen radiation followed by adjuvant alkylating agent chemotherapy. Six months after the primary treatment the normal CA 125 level increased to a pathological value during the adjuvant treatment. No tumoral progression could be documented and without changing the medication the value progressively decreased to normal values over a period of 18 months. Actually almost 5 years later the CA 125 remains normal and the patient disease free. After discussion of the different etiologic possibilities the most probably explanation seems to be a radiation-induced peritoneal inflammation. The time interval, the rapid increase, and the progressive decrease of the CA 125 level all correlate well with the histopathological appearance of chronic radiation toxicity.