OBJECTIVE This study evaluated the effect of a heparin-bonded pulmonary artery catheter (PAC) on the activated coagulation time (ACT). METHODS A prospective, controlled comparison. METHODS A tertiary care university hospital. METHODS Adult cardiac surgery patients. METHODS Celite ACTs were measured from arterial and central venous blood samples before and after the insertion of a heparin-bonded PAC. Thromboelastograms were also obtained from central venous blood samples before and 2 minutes after PAC insertion. RESULTS There was no significant difference between the sample sites before PAC insertion. After PAC insertion, the central venous ACTs were significantly increased compared with the corresponding arterial measurements at 2, 5, 10, and 20 minutes (p < 0.005, analysis of variance [ANOVA] for repeated measures, Fisher's protected least significant difference [PLSD]). The 2-minute post-PAC reaction time from the central venous blood sample was greater than 60 minutes in all cases. CONCLUSIONS The heparin-bonded PAC was associated with a localized, time-dependent alteration in the ACT. Whenever possible, blood samples for baseline ACT measurements should be obtained from an arterial catheter to minimize the anticoagulant effects from the PAC.