Estimates of glomerular filtration rate are generally obtained by measuring or estimating endogenous creatinine clearance. However, it may sometimes be difficult to obtain the necessary urine collections. Most of 19 healthy, reliable elderly outpatients were found unable to provide satisfactory 24-hour urine collections. To judge whether formulas estimating creatinine clearance from serum creatinine levels are reliable, we also compared 24-hour creatinine clearances measured in 50 inpatients with values calculated by the Cockroft-Gault equation. Only a moderate correlation was found, which may be unacceptable in the clinical situations for which the equation is used, such as drug dosing. For reasons including uncertainties in the validity of predictive formulas and unreliability of urine collections, we conclude that no acceptable method now exists for bedside estimation of glomerular filtration rate and that drug levels should be measured whenever possible in elderly patients and in those with renal insufficiency.