Sialic acid concentration in the blood serum of 104 children suffering from leukemia and malignant lymphomas was determined. A significant higher concentration of sialic acid was found at the onset of the disease, compared to the controls. In the group of children suffering from acute lymphoblastic leukemia a dependence was noted between the stage of the disease and the concentration of sialic acid in the serum. The high concentration of sialic acid at the onset of the disease, although decreasing during treatment, rose again during relapses of the disease. It seems that determination of sialic acid concentration may be useful in monitoring treatment.