Although several high-performance liquid chromatography (HPLC) methods for the determination of serum concentrations of carbapenems have been reported, they are complicated and involve column switching. We established a simple and rapid HPLC method for the determination of serum concentrations of carbapenems, and this method is suitable for routine use in the clinical field. With our HPLC method, the serum concentrations of five commercially available carbapenems could be determined by changing only the methanol/phosphate-buffer ratio in the mobile phase. Serum levels of imipenem, panipenem, and meropenem in mice could be monitored when these carbapenems (20 mg/kg) were administered subcutaneously with cilastatin (20 mg/kg). These results suggest that our simple and rapid HPLC method for the determination of the serum concentrations of carbapenems is useful for pharmacokinetic/pharmacodynamic (PK/PD)-based determination of carbapenem dosage regimens. © 2008 Japanese Society of Chemotherapy and The Japanese Association for Infectious Diseases.