Bone marrow transplantation is increasingly used to treat a spectrum of diseases in man, including immune and genetic disorders, hematological diseases, and cancer. Approximately 11,000 transplants have been performed worldwide since 1970. About two-thirds of these transplants have involved donors, including related and unrelated individuals, and in the remaining third the patient's bone marrow has been used in the form of an autotransplant. In some disorders and under carefully defined circumstances, bone marrow transplantation appears to be the preferred therapy; these diseases include aplastic anemia, acute myelogenous leukemia, acute lymphoblastic leukemia, chronic myelogenous leukemia, and selected immune and genetic disorders. In other circumstances, the value of bone marrow transplantation is less well defined. Diseases in which bone marrow transplantation may be of benefit include Hodgkin's and non-Hodgkin's lymphoma, other cancers, thalassemia, hemoglobinopathies, genetic disorders, and possibly multiple myeloma. It has been difficult to precisely identify the role of bone marrow transplantation in many of these diseases. Prospective randomized controlled clinical trials have sometimes shown an advantage for bone marrow transplantation, but in most circumstances a benefit is as yet unproven. In the U.S. the annual incidence of individuals with diseases in which bone marrow transplantation is thought to be of proven benefit is approximately 5,400, and an additional 15,000 individuals annually have diseases in which bone marrow transplantation is thought to be of possible benefit. This study reviews data available from both controlled and uncontrolled clinical trials indicating the potential role of bone marrow transplantation in the treatment of human diseases.