BACKGROUND: Surgery is the main treatment for localised renal cell carcinoma, but use of radical nephrectomy for metastatic disease is highly controversial. We aimed to establish whether radical nephrectomy done before interferon-alfa-based immunotherapy improved time to progression and overall survival (primary endpoints) compared with interferon alfa alone. METHODS: We included 85 patients from June, 1995, to July, 1998: two (one per group) were ineligible. 42 of the 83 participants were randomly assigned combined treatment (study group) and 43 immunotherapy alone (controls). All patients had metastatic renal-cell carcinoma that had been histologically confirmed and was progressive at entry. In study patients, surgery was done within 4 weeks of randomisation, and immunotherapy (5x10(6) IU/m(2) subcutaneously three times per week) started 2-4 weeks later. In controls, immunotherapy was started within 1 working day of randomisation. Follow-up visits were monthly. All analyses were by intention to treat. FINDINGS: 40 (53%) of 75 patients received at least 16 weeks of interferon-alfa treatment, which was also the median duration of treatment. Time to progression (5 vs 3 months, hazard ratio 0.60, 95% CI 0.36-0.97) and median duration of survival were significantly better in study patients than in controls (17 vs 7 months, 0.54, 0.31-0.94). Five patients responded completely to combined treatment, and one to interferon alfa alone. Dose modification was necessary in 32% of patients, most commonly because of non-haematological side-effects. INTERPRETATION: Radical nephrectomy before interferon-based immunotherapy might substantially delay time to progression and improve survival of patients with metastatic renal cell carcinoma who present with good performance status.