A total of 30 multiple myeloma patients (M=23, F=7; age 31-55 years, median 48) were allografted with peripheral blood stem cells (PBSC) from HLA-identical siblings. Time to transplantation was 3-107 months (median 8). Prior chemotherapy lines varied from 1 to 6 (median 1). Four patients were in complete remission (CR), 11 in partial remission (PR), 13 were considered to be nonresponders, and two had progressive disease. Most were conditioned with busulfan-melphalan. PBSC were collected by apheresis after G-CSF or sequential GM-CSF and G-CSF. The patients were grafted with 4.4-24.1 x 10(6)/kg CD34+ (median 7.9) and 0.9-7.9 x 10(8)/kg CD3+ cells (median 2.3). GVHD prophylaxis was methotrexate-cyclosporine. Engraftment was complete and rapid. Grades II-IV acute GVHD (aGVHD) developed in 16 (53%), but was grade III-IV only in five (17%); chronic GVHD (cGVHD) developed in 17 out of the 24 evaluable patients (71%). A total of 18 patients (71%) attained CR after transplantation. TRM was 30% overall, 16% at 100 days. There was only one relapse. Overall survival and event-free survival at 73 months were 60% and 67%, respectively. PCR negativity for IgH-gene rearrangement occurred in all persistently CR patients studied. PBSC allograft can induce long remissions, because of profound suppression of the neoplastic clone that is probably linked to the antitumor effect of cGVHD.