We conducted a randomised trial comparing an intensified versus a standard conditioning regimen for high-dose chemotherapy followed by autologous stem-cell transplantation in patients with multiple myeloma. In this study, 56 patients were randomly assigned for high-dose therapy with melphalan 200 mg/m2 or with idarubicin 42 mg/m2, melphalan 200 mg/m2 and cyclophosphamide 120 mg/kg. The primary objective was response rate. Acute toxicity, mainly because of infections, was higher in the intensified treatment arm with a treatment-related mortality of 20% versus 0% in the standard arm. This lead to the early discontinuation of the study. Response rates did not differ significantly between both treatment arms {intensified versus standard: complete response+near complete remission 50% [95% confidence interval (CI) 26-74%] vs. 33% (95% CI 17-55%), partial remission 35% (95% CI 16-61%) vs. 50% (95% CI 30-70%)}. After a follow-up of 5 years, the median time-to-progression and overall survival were not significantly different between both patient groups. Analysis restricted to patients surviving the first 100 d after transplant showed a better outcome for patients with >or=2 bad prognostic risk factors in the intensified treatment arm, however all treatment-related deaths occurred within this group of patients. In conclusion, intensified conditioning for high-dose therapy had intolerably high toxicity without improving outcome in patients with multiple myeloma.