BACKGROUND: We conducted a randomized trial of the treatment of multiple myeloma with high-dose chemotherapy followed by either one or two successive autologous stem-cell transplantations. METHODS: At the time of diagnosis, 399 previously untreated patients under the age of 60 years were randomly assigned to receive a single or double transplant. RESULTS: A complete or a very good partial response was achieved by 42 percent of patients in the single-transplant group and 50 percent of patients in the double-transplant group (P=0.10). The probability of surviving event-free for seven years after the diagnosis was 10 percent in the single-transplant group and 20 percent in the double-transplant group (P=0.03). The estimated overall seven-year survival rate was 21 percent in the single-transplant group and 42 percent in the double-transplant group (P=0.01). Among patients who did not have a very good partial response within three months after one transplantation, the probability of surviving seven years was 11 percent in the single-transplant group and 43 percent in the double-transplant group (P<0.001). Four factors were significantly related to survival: base-line serum levels of beta2-microglobulin (P<0.01) and lactate dehydrogenase (P<0.01), age (P<0.05), and treatment group (P<0.01). CONCLUSIONS: As compared with a single autologous stem-cell transplantation after high-dose chemotherapy, double transplantation improves overall survival among patients with myeloma, especially those who do not have a very good partial response after undergoing one transplantation. Copyright 2003 Massachusetts Medical Society