We performed the first successful syngeneic bone marrow transplantation (BM Txp) in a patient with multiple myeloma. The patient and his normal identical twin are 50-year-old physicians. Prior to BM Txp, a partial remission was achieved with 1 year of continuous low dosage melphalan and prednisone therapy. Immediately before BM Txp, high dose cyclophosphamide and total body irradiation were administered in an attempt to eradicate the residual tumor. For 17 months after BM Txp, the patient was asymptomatic and hematologically normal although a low concentration of serum monoclonal IgGκ persisted. In the 18th month, recurrence of bone pain and increase in the monoclonal IgG signalled exacerbation of the disease. Chemotherapy was resumed and again produced objective and subjective evidence of response. This study demonstrates the feasibility and potential usefulness of syngeneic BM Txp in myeloma.