The renal function of 15 patients receiving cis-platinum (II) dichlorodiammine (CPDD) was examined prospectively in detail to elucidate early evidence of nephrotoxicity. Patients were given a total of 49 couses of CPDD at 20 mg/m2/day for 5 days with 1,000 ml of saline prehydration. Renal function was monitored by serial determinations of serum creatinine and glomerular filtration rate (measured as 125I-iothalamate clearance) and by measurement of parameters of tubular function, including tubular reabsorption of phosphorus, urine-to-serum glucose ratio, total protein, and total free immunoglobulin light chain excretion, serum electrolytes, and urine pH and specific gravity. There was no significant change in mean serum creatinine within a course of treatment, nor was there a cumulative increase in the serum creatinine. In 9 of 19 evaluable courses there was a small transient fall in glomerular filtration rate with prompt recovery. There was no cumulative decrease in glomerular filtration rate through 3 courses of treatment. Four of the patients with preexisting renal insufficiency suffered no significant additional nephrotoxicity. There was no tubular dysfunction demonstrable in any of the patients. This study represents the first prospective detailed examination of multiple parameters of renal function in patients treated with CPDD and reveals that the only parameter to show any change with this schedule of drug administration was the glomerular filtration rate.