Hematopoietic stem cell transplantation is being successfully used to treat a variety of malignant and nonmalignant disorders. This therapy has resulted in an increasing number of survivors who are at risk for adverse long-term outcomes, including the development of second and subsequent malignant neoplasms. We review the incidence and spectrum of posttransplantation malignancies and discuss risk factors and future directions for research. (C) 2004 The Japanese Society of Hematology.