A simple and fast HPLC-UV method is described for the simultaneous determination of total or free ceftazidime and avibactam in serum, which is suitable for therapeutic drug monitoring (TDM) or pharmacokinetic studies in man. Sufficient retention of the very polar avibactam was obtained by addition of tetrabutylammonium hydrogen sulfate (TBA) as ion pairing agent, thus allowing the determination of both drugs in serum by isocratic elution. Total concentrations were determined after protein precipitation with acetonitrile, free concentrations after ultrafiltration. Separation was performed using an XBridge BEH C18 column with a mobile phase consisting of 20 mM sodium phosphate buffer/acetonitrile 90:10 (v/v), pH 6.5, containing 5 mM TBA. The lower limit of quantification was 1 mg/L for ceftazidime and 0.5 mg/L for avibactam, respectively. The imprecision of the determination of total drug was <3 %, the accuracy between 99.0 % and 104 %. Determination of free drug in quality control samples resulted in unbound fractions of 97.9 ± 2.0 % for ceftazidime and 99.4 ± 3.2 % for avibactam.