Between January 1989 and November 1995, 259 patients with multiple myeloma (MM), 22 stage I, 57 stage II and 180 stage III at diagnosis were treated with myeloablative high-dose therapy followed by autologous peripheral blood stem cell (PBSC) transplantation. The median time from diagnosis to transplantation was 17 months (6-112). At the time of transplant, 56 patients were in CR, 153 in PR, 25 were nonresponders and 25 had progressive disease. Mobilization of stem cells was performed with G-CSF alone in 141 cases, chemotherapy plus G-CSF in 65, chemotherapy plus GM-CSF in 36 and chemotherapy alone in 17 patients. The conditioning regimen consisted of high-dose melphalan alone in 96 patients, melphalan plus TBI in 73, busulfan plus melphalan in 56, busulfan plus cyclophosphamide in 27 and cyclophosphamide plus TBI in seven. The median durations of neutropenia (>0.5 x 10(9)/l) and thrombocytopenia (>20 x 10(9)/l) were 12 (5-118) and 13 days (5-360), respectively. Transplant-related mortality occurred in 11 patients (4%). Once a stable graft was achieved, 114 patients (44%) received maintenance treatment with recombinant alpha interferon (IFN-alpha). Among the 248 patients evaluable for response 125 (51%) had a CR and 100 had a PR (40%). The median duration of progression-free survival (PFS) and overall survival (OS) after transplantation was 23 and 35 months, respectively. Univariate analysis showed that response status pretransplant, only one line of primary induction treatment and IFN-alpha maintenance treatment post-transplant significantly influenced OS. Female sex, pretransplant responsive disease, and treatment with IFN-alpha post-transplant were the factors significantly influencing PFS. The conditioning regimen and method of stem cell mobilization had no significant impact on OS and PFS. On multivariate analysis the only independent factors associated with a longer survival were the number of chemotherapy courses prior to autologous PBSC transplantation and the pretransplant response status. The present analysis from the Spanish Registry confirms the feasibility of autologous PBSC transplantation in myeloma patients with a very low toxicity (4% toxic deaths). The high complete response rate after transplantation is encouraging. The best results are obtained when the procedure is performed early after the first line of induction therapy and in patients with chemosensitive disease. Whether early high-dose therapy followed by autotransplantation in responding patients is superior to conventional chemotherapy is currently being investigated in prospective randomized studies.