The validity of the Cockroft and Gault formula in monitoring renal function during cisplatin therapy was examined in 110 consecutive cancer patients. A good correlation between estimated creatinine clearance (ECC) and the standard measured creatinine clearance (MCC) was found both before and after the onset of cisplatin. The correlation was better in males (r = 0.78 prior to cisplatin and r = 0.79 after the initiation of cisplatin) than in females (r = 0.69 and r = 0.63 respectively). Obesity (> 125% of ideal body weight) had no major effect on the correlation between ECC and MCC. The presence of third-space body fluids, on the other hand, had an influence on the relations between ECC and MCC, particularly in females (r = 0.49). It is concluded that estimation of creatinine clearance, without the cumbersome collection of 24-h urine specimens, is accurate enough for the purpose of adjusting cisplatin dose. It should not be used in females with third-space body fluids.