['To determine the relationships among serum aminoglycoside clearance, renal aminoglycoside clearance, measured creatinine clearance, and estimated creatinine clearance derived from a standard formula in critically ill patients.', 'A ten-bed general ICU in a university hospital.', 'Eighteen critically ill patients who were being treated with gentamicin or tobramycin for severe infections, and were hemodynamically stable.', 'The various clearances were measured simultaneously after the administration of a dose of aminoglycoside by assaying serial blood samples for aminoglycoside and creatinine concentration, and by measuring the content of these substances in urine collected over the same time period.', 'The slopes, intercepts and coefficients of determination (r2) of the various regressions were determined, along with the 95% confidence intervals for the prediction of serum aminoglycoside clearance from each other variable.', 'Renal aminoglycoside clearance, creatinine clearance, and estimated creatinine clearance accounted for only 58%, 59%, and 62%, respectively, of the variance in serum aminoglycoside clearance. Only 64% of the variance in renal aminoglycoside clearance was explained by creatinine clearance. Substantial and variable nonrenal aminoglycoside clearance was evident.'] Is serum aminoglycoside clearance predicted as poorly by renal aminoglycoside clearance as by creatinine clearance in critically ill patients?