Fifty-seven cancer patients, who had had an 8–15% loss of body weight in the previous 6 months, were studied to establish the reliability in wasted individuals of presently available nomograms correlating serum creatinine with creatinine clearance. Other predictors of renal function, such as osmolar clearance, free water clearance, and excreted fraction of filtered sodium, were also considered. In all patients, and especially in those with a measured creatinine clearance 10–50 ml/min, the use of standard nomograms based on serum creatinine resulted in gross overestimation of renal function. None of the other tests of renal function proved reliable for clinical purposes. It is concluded that in wasted patients administration of nephrotoxic drugs that are renally excreted should be adjusted to measured creatine clearance. Using other common predictors of renal function may result in overdosage.