Thirty-five adults with acute leukemia were studied to determine the concentration of the serum ferritin. The results indicate that the level of serum ferritin in patients with acute leukemia was elevated and that the difference among the subtypes of acute leukemia was not significant. Seven patients were monitored with serial determination of serum ferritin during the couse of treatment. It was discovered that the decreased number of the blasts in bone marrow was coincided with a fall in the serum ferritin concentration. The high levels .of the serum ferritin may given the signal for the active disease in adult acute leukemia. The possible causes of the elevated serum ferritin were discussed ’