Inulin and para-aminohippuric acid clearances, determined by the conventional method of continuous intravenous infusion with blood and urine sample collections, are the gold standards for estimating glomerular filtration rate and renal plasma flow, respectively. Creatinine clearance provides a reasonably good estimate of glomerular filtration rate but is still subject to errors in accuracy and precision. However, novel methods employing cimetidine to block renal tubular creatinine secretion hold promise for improving the accuracy of estimates. More importantly, a large (and growing) number of studies have consistently demonstrated that estimating glomerular filtration rate by creatinine clearance calculated from the Cockcroft-Gault formula is better than measuring creatinine clearance with a 24-h urine collection. Until newer, more simple methods are developed, calculating creatinine clearance using fasting serum creatinine level, body weight, age and sex provides a reasonable and clinically useful bedside measure of glomerular filtration rate for the practising clinician.