Urine and serum ferritin levels were measured by a double-antibody radio-immunoassay in 96 normal adults and 57 patients with urologic malignancies. In the 96 adults, the urine ferritin levels ranged from 0 to 28 ng/ml. Elevated urine ferritin levels were observed in all 7 patients with renal pelvic carcinoma, 4 of 13 patients with renal carcinoma, 1 of 13 patients with well-differentiated (Grade I) bladder carcinoma, 8 of 19 patients with median-differentiated (Grade II) bladder carcinoma, and all 5 patients with poorly-differentiated (Grades III, IV) bladder carcinoma. These results indicated that urine ferritin assay was helpful in discriminating well-differentiated bladder carcinoma from the poorly-differentiated (P less than 0.01) in addition to differentiating renal pelvic carcinoma from renal carcinoma (P less than 0.05).