Piperacillin inactivation of the aminoglycosides isepamicin and gentamicin in 12 chronic hemodialysis patients was assessed. Six subjects each received isepamicin (7.5 mg/kg of body weight) or gentamicin (2 mg/kg) alone and in combination with piperacillin (4 g every 12 h for four doses). Isepamicin and gentamicin concentrations in plasma and urine were monitored over 48 h after each dose and analyzed by high-performance liquid chromatography and fluorescence polarization immunoassay, respectively. The pharmacokinetics of isepamicin were not significantly altered during combination treatment with piperacillin. The total body clearance (3.79 ± 0.71 versus 3.94 ± 1.05 ml/min), the steady-state volume of distribution (0.19 ± 0.04 versus 0.18 ± 0.03 liter/kg), and the terminal elimination half-life (47.91 ± 7.20 versus 45.08 ± 10.34 h) were not significantly altered in the presence of piperacillin. In contrast, the terminal elimination half-life (47.68 ± 20.58 versus 35.67 ± 11.18 h) of gentamicin was significantly reduced when gentamicin was given with piperacillin. The total body clearance (4.26 ± 3.07 versus 4.89 ± 1.94 ml/min) and the steady-state volume of distribution (0.19 ± 0.04 versus 0.20 ± 0.04 liter/kg) of gentamicin were not significantly altered during combination therapy; however, the nonrenal clearance of gentamicin administered in combination with piperacillin (3.56 ± 0.38 ml/min) increased significantly compared with that of gentamicin (2.03 ± 0.50 ml/min) given alone. The results of this study suggest that no additional dosage adjustment of isepamicin during concomitant therapy with piperacillin in hemodialysis patients is necessary. However, this does not preclude the need for appropriately ex vivo-handled specimens for monitoring isepamicin concentrations in plasma to ensure therapeutic efficacy and prevent toxicity. Furthermore, additional dosage adjustments may be necessary when gentamicin is used concomitantly with piperacillin, on the basis of the significant in vivo inactivation that takes place in end-stage renal disease patients.