In patients with renal failure the dose of renally excreted drugs should be reduced in proportion with the degree of impairment of glomerular filtration: in this situation it is appropriate to measure the glomerular filtration rate in order to determine accurately the correct dose. However, as 24 hour urine based creatinine clearance measurements are time consuming and inaccurate, the parameter most often used to estimate renal function in clinical practice is the serum creatinine concentration. An isolated serum creatinine concentration is unsatisfactory for this purpose as it depends on creatinine production, which is related to muscle mass, as well as on renal elimination of creatinine. A variety of formulae and nomograms have been devised to assist clinicians in predicting the glomerular filtration rate from a stable serum creatinine concentration and the patient's sex, age and weight (or height). Although some of these formulae have been available for over 15 years, the majority of doctors cannot estimate accurately the glomerular filtration rate from these parameters. It is proposed in this paper that biochemistry results computers should employ one of these formulae to calculate the glomerular filtration rate/70 kg, using the age and sex information provided on the request form, each time a serum creatinine concentration is reported. These formulae are invalid in several well defined clinical situations which could be briefly outlined on the report.