Most chemotherapy agents function by causing damage to the DNA of rapidly dividing cells, such as those in the bone marrow, leading to anemia and leukopenia during chemotherapy and the development of secondary leukemias in the years following recovery from the original disease. We created an animal model of nitrosourea-based chemotherapy using ethylnitrosourea (ENU) to investigate the effect of niacin deficiency on the side effects of chemotherapy. Weanling Long-Evans rats were fed diets containing various levels of niacin for a period of 4 weeks. ENU treatment started after 1 week of feeding and consisted of 12 doses delivered by gavage, every other day. Cancer incidence was also monitored in the following months. ENU treatment caused many of the acute symptoms seen in human chemotherapy patients, including anemia and neutropenia. Niacin deficiency (ND) had several interesting effects, alone and in combination with ENU. Niacin deficiency alone caused a modest anemia, while in combination with ENU it induced a severe anemia. Niacin deficiency alone caused a 4-fold increase in circulating neutrophil numbers, and this population was drastically reduced by ENU-treatment. In the long term, macin deficiency caused an increased incidence of cancer, especially chronic granulocytic leukemias.