Based on the linear single-compartment pharmacokinetic consideration of endogeneous creatinine a new equation is derived to estimate the true creatinine renal clearance and hence the renal function of a patient; By using this equation one can estimate clinically the creatinine renal clearance of a patient simply by measuring the creatinine serum levels at two different times without the necessity of urine collection. Almost a perfect prediction of the status of renal function in two anephric patients was demonstrated using this proposed methodmwith a sensitive assay technique for endogeneous creatinine, the proposed equation would enable on to monitor the creatinine clearance on an hourly basis= The method is especially valuable to monitor the rapidly changing (improving or deteriorating) renal function in all types of patients and also in certain groups of patients whose urine is difficult to ba accurately collected;