Charts of 122 patients with megaloblastic anemia due to either B12 or folic acid deficiency were reviewed for the presence of fever. Fever, defined as a temperature of 100 F (37.8 C) or more, was present in approximately 40% of the patients with both. The elevation was usually minimal but was sometimes over 104 F (40 C), usually in those with more severe anemia and thrombocytopenia, and greater elevations of SGOT and bilirubin concentration. In uncomplicated cases, the temperature returned rapidly to normal after adequate vitamin therapy. Failure of the fever to disappear rapidly with treatment should suggest the probability of some cause other than megaloblastic anemia.