Metabolic diseases that have been treated by bone marrow transplantation (BMT) can be divided into three groups: (1) group I, those diseases in which the clinical symptomatology is restricted to cells of the lymphohematopoietic system, (2) group II, those diseases in which nonlymphohematopoietic cells are clinically affected but the central nervous system (CNS) is not affected, and (3) group III, those diseases in which the CNS is affected (Table 1). Group I diseases are all correctable by BMT, group II diseases may be correctable depending upon the biology of the primary disease, and group III diseases are still the center of controversy.