The creatinine clearance was calculated in standard fashion from a timed urine specimen (measured creatinine clearance [MCC]) and from a previously published formula (estimated creatinine clearance [ECC]), in 55 instances, in 19 consecutive patients who were admitted to the hospital for treatment with cisplatin. Using creatinine excretion as an index of completeness of urine collection, there were 19 (35%) inaccurate collections. The correlation between creatinine clearances calculated by both methods was excellent (r = 0.684, P less than .001) and improved when inaccurate collections were excluded (r = 0.922, P less than 0.001). A discrepancy between the two methods of 25% or more was found in 19 collections. Using data from patients with two or more collections to test whether or not the two methods produced equally variable results, indicated that the MCC is a more variable, less reliable method than the ECC. In eight of 55 urine collections, the results of MCC were not used as a guide to chemotherapy and, in an additional 16 should have not been used because of inaccuracy in the urine collection. These results suggest that the creatinine clearance as calculated by an alternative method (ECC) should replace the use of MCC when assessment of the renal function is needed before the administration of nephrotoxic agents.