A 51-year-old woman had acute myelogenous leukemia following log-term cyclophosphamide therapy for rheumatoid arthritis. After standard methods of management had failed, the rheumatoid arthritis showed considerable improvement in response to approximately 25 mg cyclophosphamide per day over a four-year period. At the end of this period, pancytopenia developed, and cyclophosphamide was discontinued. A bone-marrow aspirate showed nonspecific changes. However, four months later because of severe progressive pancytopenia, another bone-marrow examination was performed; it showed acute myelogenous leukemia. Therapy failed to induce a remission, and two months after diagnosis the patient died of aspergillosis. Autopsy confirmed the presence of leukemic infiltration of bone marrow, lymph nodes, liver and spleen. These findings suggest a possible leukemogenic role of cyclophosphamide in this case and suggest that caution should be exercised in treating non-fatal diseases with antitumor drugs.