The pharmacokinetics of gentamicin in adult patients with endocarditis were studied. The records were reviewed for 64 patients treated for bacterial endocarditis and for whom serum gentamicin concentrations had been requested between May 1990 and May 1993. The patients were divided into those with serum creatinine concentration (SCr) <1.2 mg/dL and those with SCr > or = 1.2 mg/dL. The measured serum gentamicin concentrations, patient demographic information, dosage interval, and SCr were entered into a pharmacokinetics program for analysis. The pharmacokinetic values evaluated were steady-state distribution volume (V), clearance (CL), elimination rate constant (k), and half-life (t1/2). The mean +/- S.D. t1/2 and V of gentamicin were 4.7 +/- 2.4 hr and 0.29 +/- 0.11 L/kg. Half-life was correlated with SCr and V. These two variables may explain 66% of the variation in t1/2. No difference in V was observed between patients with normal versus abnormal SCr. Men had a lower k than women. V and CL were lower in patients under age 60 than in older patients. Adult patients with endocarditis may have expanded gentamicin V. V and CL accounted for most of the variation in gentamicin t1/2.