The purpose of this study was to describe the population pharmacokinetics of gentamicin in a group of 939 adult hospitalized patients receiving once-daily administration of gentamicin and to evaluate the potential influence of patient covariates on gentamicin disposition. Data comprising 1294 serum concentrations from 939 patients, were analyzed using a nonlinear mixed-effect model (NONMEM). The patients had an average age of 55 and an average weight of 70 kg, 431 of the patients were female. The patient covariates including body weight, gender, age, and creatinine clearance (CL(CR)) were analyzed in a stepwise fashion to identify their potential influences on gentamicin pharmacokinetics. The data were best described with a two-compartment model. NONMEM analyses showed that gentamicin clearance (CL, l/h) was linearly correlated with CLcR with proportionality constant: 0.047 (S.E.: 0.0035) x CL(CR) (ml/min). Volume of the central compartment (V1, 1) was linearly related to body weight with proportionality constant: 0.28 (S.E.: 0.021) x body weight (kg). The mean population estimates of CL and V1 were 4.32 l/h and 19.61. respectively. The inter-individual variability in CL and V1 were 29.6 and 5.8%, respectively. Residual errors were 0.23 mg/l and 23.7%. The mean population values of CL and V1 of gentamicin dosed once daily are in agreement with those described by others. This analysis indicates that once-daily dosing (7 mg/kg) of gentamicin should achieve satisfactory concentration in patients with normal renal function although serum concentration monitoring is required to confirm the optimal dosing interval in patients with impaired renal function.