The influence of renal insufficiency on melphalan-induced myelosuppression was examined during the initial 10 weeks of treatment in 295 patients with multiple myeloma. Patients were randomized to receive either oral melphalan (0.15 mg/kg/day for 7 days, followed by 0.05 mg/kg/day after recovery from the wbc count nadir) or iv melphalan (16 mg/m2 every 2 weeks for four doses, followed by a single dose every 4 weeks). All patients received a 6-week tapering course of prednisone. Patients with renal insufficiency (BUN greater than or equal to 30 mg/100 ml) had a significantly higher frequency of severe leukopenia (less than or equal to 1000 cells/mm3) following iv melphalan than did patients with normal renal function (50% vs 15%, respectively; P = 0.007). The latter effect resulted in an increased frequency of drug-related deaths secondary to infection. The frequency of severe thrombocytopenia (less than or equal to 25,000 cells/mm3) was also greater in patients with renal failure following iv melphalan therapy. Reduction of iv melphalan dose to 50% in patients with elevated BUN reduced the frequency of these complications to levels that were not significantly different from those observed in patients with normal renal function. The frequency of severe myelosuppression was independent of renal function in patients receiving oral melphalan. Possible explanations for these findings are discussed.