In order to evaluate the clinical significances of the serum vitamin $B_{12}$, folate and ferritin levels in patients with malignant tumors, the levels were measured in 10 normal contol 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 $B_{12}$, folate and ferritin level were $588.80{pm}131.58pg/ml,;5.59{pm}1.52ng/ml;and;89.22{pm}42.78ng/ml$ retrospectively 2) There was no significant difference in serum levels between patients with benign diseases and normal control subjects. 3) The serum vitamin $B_{12}$ 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 $B_{12}$ 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 $B_{12}$ and ferritin may be useful as tumor markers in patients with malignant tumors.