The population pharmacokinetics of levofloxacin (LVFX), the l-enantiomer of ofloxacin, were studied in 522 subjects, including normal subjects and patients with infectious diseases. Altogether, 1572 LVFX serum concentrations were obtained following a single oral administration during clinical trials. The influences of renal function, age, meals and concurrent drug administration on the pharmacokinetic parameters of LVFX were examined by the likelihood ratio test using a nonlinear mixed-effect model (NONMEM). In patients with renal insufficiency, the total body clearance (CL) of LVFX was related to creatinine clearance (Ccr) and body weight (WT), as expressed by CL = 0.0836 Ccr + 0.013 WT. The CL with normal renal function was 0.178 (l/h/kg). The apparent volume of distribution (Vd) was calculated to be 1.46 (l/kg). The elderly subjects, aged 65 years old or over, exhibited, on average, a 32% reduction in CL and 6% greater Vd. LVFX was rapidly absorbed in the gastrointestinal tract, but the concurrent administration of antacids (magnesium, aluminum, etc.) decreased the bioavailability of LVFX by 15-52%. The recommended dose of LVFX was decided on the basis of current pharmacokinetic information and the minimum inhibitory concentrations. Population pharmacokinetic parameters are also useful for the individualization of a dosage regimen by means of the Bayesian forecasting method.