The present study examined the pharmacokinetics of biapenem during continuous venovenous hemodiafiltration (CVVHDF) and assessed the pharmacodynamic exposure, based on a pharmacokinetic model, to consider biapenem dosage adaptation in CVVHDF. Biapenem (300 mg) was administered by 2-h infusion to seven critically ill patients receiving CVVHDF. The flow rates were 60 ml/min for blood, 800 ml/h for filtrate, and 600 ml/h for dialysate. The drug concentrations in plasma and filtrate-dialysate were determined by high-performance liquid chromatography and analyzed pharmacokinetically. The sieving coefficient was 0.92 +/- 0.06 (mean +/- SD). The simulation curves, using a multicompartment model, were well fitted to the measurements in plasma and filtrate-dialysate. The clearance by CVVHDF and the clearance by non-CVVHDF routes were 1.29 +/- 0.08 and 6.14 +/- 1.89 l/h, respectively. The multicompartment model was used to assess the pharmacodynamic exposure (time above the minimum inhibitory concentration of 4 microg/ml) in plasma. When the total daily dose was 600 mg, the duration of time was greater at 300 mg every 12 h than at 600 mg every 24 h. The minimum dosages needed to achieve more than 30% of the dosing interval at filtrate-dialysate flow rates of 1.4, 2.8, and 5.6 l/h were 300 mg every 12 h, 600 mg every 12 h, and 600 mg every 12 h, respectively. These results suggested that low doses or increased dosing intervals should be avoided in patients receiving this renal replacement technique. Information on pharmacodynamic exposure obtained from this model may help us to determine the appropriate biapenem dosage for CVVHDF. Moreover, our pharmacokinetic model may be useful for further pharmacokinetic studies of biapenem.