Sequential assessment of renal function in 50 critically ill postoperative patients was done by simultaneous determinations of osmolar, free water, creatinine and sodium clearances, and fractional excretion of sodium. The traumatic and nontraumatic critically ill surgical patients had a characteristic pattern of creatinine clearance (Ccr) which was age related. This was manifested in the trauma patient by Ccr which were initially greater than expected (140–190 ml/min 1.73 M2). In the uncomplicated postoperative patients, a positive correlation was found between increased cardiac index and the indexed glomerular filtration rate. Decreases in free water clearance and Ccr were useful in predicting the onset of renal failure before the development of the oliguric state. The authors conclude that frequent assessment of those renal functions which are most adversely affected by accidental or surgical trauma is most appropriately monitored by serial measurement of creatinine and free water clearance.