In order to evaluate the clinical significances of the serum vitamin B12, folate and ferritin levels in patients with malignant tumors, the levels were measured in 10 normal control subjects, 70 patients with malignant tumors, 7 patients with liver cirrhosis and 25 patients with other benign diseases. The results are as follows: 1) In normal control subjects, mean serum values for vitamin B12, folate and ferritin level were 588.80¡¾ 131.58 pg/ml, 5.59¡¾1.52 ng/ml and 89.22¡¾42.78 ng/ml retrospectively. 2) There was no significant difference in serum levels between patients with benign diseases and normal control subjects. 3) The serum vitamin B12 and ferritin levels in patients with liver cirrhosis were significantly higher than in normal control, and the serum folate levels in these patients were lower than in normal control subjects. 4) The serum vitamin B12 and ferritin levels in patients with malignant tumors were significantly higher than in normal control subjects, and the serum folate levels in these patients were significantly lower than in normal control subjects. The above results suggest that the serum vitamin B12 and ferritin may be useful as tumor markers in patients with malignant tumors.