We studied the rate of disappearance of neutrophil hypersegmentation in 23 patients with megaloblastic anemia during therapy with vitamin B12 and folic acid. In 14 patients with uncomplicated megaloblastic anemia, the neutrophil lobe average began to fall towards normal 11.4 ± 0.3 (SEM) days after treatment was begun and became normal by 13.9 ± 0.6 days. In nine patients with associated inflammatory disorders, the lobe average returned to normal more rapidly. In six initially neutropenic patients, a five-fold increase in total granulocyte count occurred by the eighth day and a sevenfold increment in the total number of circulating hypersegmented neutrophils. Granulocyte folate concentrations in five folate-deficient patients rose to normal levels within 4 to 7 days of starting therapy. Hypersegmentation thus persists for many days after correction of neutropenia and restoration of normal granulocyte folate levels have occurred. We conclude that cobalamin and folate are needed at two different steps in granulopoiesis.