Vitamin B12 deficiency can cause profound alterations in the bone marrow. These alterations can mimic the more serious diagnosis of acute leukemia. The two patients described in this report were originally suspected of having acute leukemia or myelodysplasia on the basis of the bone marrow smear, and induction chemotherapy was considered. However, after further studies, they were both found to have vitamin B12 deficiency, and parenteral vitamin B12 administration resulted in normalization of the bone marrow.