This study describes the occurrence of renal failure among 1353 newly diagnosed cases of multiple myeloma. Renal function was evaluated by serum creatinine concentration in 1353 cases, 31% of whom had renal failure at the time of diagnosis. In 1206 cases an estimation of creatinine clearance was made. When renal failure was defined by using creatinine clearance estimation, 49% had renal failure at the time of diagnosis. Renal failure was present in 24% of patients with an M component of IgG-, 31% of IgA- and 100% of IgD-type. 52% of patients with light chain disease had renal failure. The frequency of renal failure was similar in lambda- and kappa-light chain disease. Patients with a high excretion of Bence Jones protein in the urine (> 10 g/24 h) had renal failure significantly more often than patients with lower excretion. Renal failure was related to advanced disease; 41% of patients with stage III (Durie-Salmon) disease had renal failure. Renal failure was found in 45% of patients with hypercalcaemia. When estimated creatinine clearance was used as a predictor of renal function, the same trends were found as mentioned above. In addition, the proportion of patients with renal failure was found to increase with advancing age.