Serum ferritin levels were determined in 75 hematologic, 1205 malignant, and 538 benign diseases. In hematologic diseases the serum ferritin level was generally low in iron deficiency anemia, but high in hemochromatosis, aplastic anemia, and leukemia. In malignant diseases the serum ferritin concentration was increased remarkably in lung, liver, biliary, and urogential diseases and in malignant lymphoma. Among benign diseases hepatitis cases showed a tendency for high serum ferritin levels. Serum ferritin levels are influenced by various factors, and their careful evaluation may be essential for understanding of the underlining disease process.