Gentamicin pharmacokinetics were determined in 26 critically ill adult patients. All had normal or only moderately reduced renal function based on serum creatinine concentration and clearance. The average half-life of elimination was 4.68 h, creatinine clearance was 3.9 l/h, and gentamicin clearance was 2.86 l/h. There was a highly significant correlation between the area under the serum concentration-time curves (AUC) and the peak and trough concentrations (r = 0.908, 0.980, P < 0.05, respectively). Serum gentamicin concentrations rose rapidly in 10 patients who showed deterioration of kidney function after 5 days of therapy. Gentamicin peak concentrations increased by 31%, while trough concentration increased by 121%. The mean percentage change in half-life was 68.49%, while the mean percentage change in renal function was 44.38%. Linear regression analysis of AUC versus the percentage change in kidney function revealed a significant increase in AUC with decrease in kidney function (r = 0.909, P < 0.05). There was a significant increase in half-life with decrease in kidney function (r = 0.948, P < 0.05).