The performance of three samples each of six brands of flow-directed pulmonary artery (PA) catheters was evaluated. In vivo studies determined the reproducibility of PA waveforms over a range of controlled heart rates when compared to those obtained from a transducer-tipped catheter. In vitro studies determined their frequency response, thermistor accuracy, balloon characteristics, and the maximum infusion rates possible through their proximal lumens. All catheters performed similarly in vivo, with mean errors in systolic and diastolic pressures ranging, respectively, from +9% to -12%, at a HR of 80 beat/min, to +23% and -30% at a HR of 160 beat/min. The resonant frequencies of all catheters in vitro were similar at 12 Hz or less; we were unable to obtain the frequency responses (17 to 32 Hz) claimed by the manufacturers. All thermistors were accurate for clinical use when tested over the temperature range 32 degrees to 42 degrees C. Balloon characteristics and infusion rates varied with some catheter brands. Maximum infusion rates varied from 164 to 383 ml/h when infusing 20% dextrose, and from 95 to 213 ml/h when infusing 35% dextrose.