To determine the validity of endogenous creatinine clearance as a measure of glomerular filtration rate in patients with renal disease 308 simultaneous determinations of inulin clearance and endogenous creatinine clearance were reviewed and analysed. The ratio of creatinine clearance/inulin clearance increased progressively with the decline in inulin clearance, while the finding of a normal endogenous creatinine clearance masked a definite but mild decrease of glomerular filtration rate in 42% of the patients and a moderate decrease of glomerular filtration rate in 23%. This indicates that with declining glomerular filtration rates the endogenous creatinine clearance progressively overestimates actual glomerular filtration rates. Hence a single determination of creatinine clearance can be misleading as a screening measurement of glomerular filtration rate.