The thrombogenicity of heparin-bonded and non-heparin-bonded pulmonary-artery catheters was compared by means of pull-out phlebography in a prospective, randomized clinical study of 20 patients in an intensive care unit (ICU). The duration of catheterization averaged 47.4 +/- 4.1 h in the heparin-bonded group (10 patients) and 45.7 +/- 7.1 h in the non-heparin-bonded (10 patients). Age, clinical diagnosis, mean time required to obtain a stable wedge position, heparin dose administered and clotting indices were comparable between the groups. There was no statistically significant difference in the length of formed thrombi (2.3 +/- 1.89 vs 5.88 +/- 4.62 cm) or frequency of thrombus formation between the groups. The thrombi originated and propagated from the puncture site. They were mural proximally and formed sleeves in the superior vena cava. Several sleeves stripped off on withdrawal of the catheter. Protection from thrombosis offered by heparin bonding of pulmonary-artery catheters should, therefore, not be overestimated--at least not in long-term catheterization of ICU patients.