Serum ferritin levels were determined by a radioimmunoassay (a 2-site solid-phase method). The mean ferritin level for 52 control women was 35 ng/ml, and the 2nd to 98th percentile reference intervals were 3.9 to 108 ng/ml. The upper limit of ferritin level for controls was determined to be 108 ng/ml, and the sera from cervical cancer patients were screened. In 98 patients with untreated cervical cancer, 50 (51%) had elevated levels of serum ferritin. Among 36 patients who were surgically treated and whose resected specimens were pathologically examined, 12 of 16 patients (75%) with ferritin levels above normal and 3 of 20 patients (15%) with levels below 108 ng/ml had parametrial invasion and/or lymph node metastasis. Serum ferritin levels were measured at 2- to 4-week intervals and 6 months after treatment on 21 patients with levels elevated before therapy. Ferritin levels decreased to the normal range 4 weeks after treatment in many patients, but the development of elevated levels thereafter was closely associated with a poor prognosis. Measurement of serum ferritin may be useful in predicting the extent of cancer beyond the cervix and the prognosis for cervical cancer patients.