Between November 1998 and October 1999 authors treated five multiple myeloma patients with an allogeneic peripheral blood stem cell transplantation from HLA-identical sibling using a non-myeloablative conditioning regimen. The median age at the time of transplantation was 58 (range: 47-65) years. In all patients one (n = 3) or two (n = 2) autologous peripheral blood stem cell transplantations were already performed. Conditioning was performed with fludarabine, oral busulfan and anti-T-lymphocyte globulin. All patients engrafted from 13 to 18 (median: 17) days from transplantation. The duration of neutropenia (absolute neutrophiles count < 500/microl) and thrombocytopenia (platelets < 20,000/microl) ranged between 4 and 19 (median: 18) and between 13 and 18 (median: 17) days, respectively. In the period of posttransplant pancytopenia two patients developed mild gastrointestinal mucositis and two pulmonary complications (bronchopneumonia and dyspnoe of unknown etiology). Two patients had grade III-IV acute graft-versus-host disease (GvHD), none had extensive chronic GvHD. Two patients received prophylactic donor-lymphocytes infusions 200 and 225 days from transplantation. One of them developed grade III acute GvHD. All patients responded. One achieved complete and four partial remission of the disease. One patient died 111 days from transplantation due to bronchopneumonia, four are alive and well, in the stable disease, 35, 36, 51 and 52 weeks after transplantation. It can be concluded that allogenic peripheral blood stem cell transplantation using a non-myeloablative conditioning regimen is an effective way of the multiple myeloma treatment with an acceptable toxicity.