The purpose of the present study was to analyse the importance and prognostic value of renal failure in multiple myeloma patients. The frequency and reversibility of renal failure in 775 multiple myeloma patients diagnosed between 1984-86 and 1990-92 in the Nordic countries were studied. Renal failure, defined as plasma creatinine > 130 micromol/l, was observed in 29% of the cases at the time of diagnosis. During the first year after diagnosis 58% achieved normalisation of p-creatinine, and this was achieved mainly during the first 3 months. Reversibility of renal failure was more frequently observed in patients with moderate renal failure, hypercalcaemia and low Bence-Jones protein excretion. In a multivariate analysis renal failure, high age, stage III disease and hypercalcaemia were independent prognostic factors for survival. Patients who needed dialysis had a poor prognosis, with a median survival of 3.5 months. A 12-months landmark analysis showed that reversibility of renal failure was a more important prognostic factor than response to chemotherapy. It is concluded that renal failure in multiple myeloma is reversible in about half the cases, and reversibility of renal failure improves long-term survival.