Activated clotting time (ACT) and activated partial thromboplastin time (APTT) are used for monitoring heparin therapy during coronary angioplasty. The purpose of this study was to determine which parameter is more useful clinically, and to assess the correlation between ACT and APTT. The authors measured these parameters at fixed intervals (0, 15, 30, 60, 150 and 240 mins) following intravenous heparin administration (12,500 +/- 3100 U) during coronary intervention in 39 patients. APTT rose to 'therapeutic' levels (true therapeutic levels have not been defined for these coagulation tests) more rapidly and fell to subtherapeutic levels more slowly than did ACT. The combination of a subtherapeutic ACT with a therapeutic APTT occurred far more often than did concordant therapeutic ACT and APTT values (69% versus 31%, P < 0.0004). There was a relatively poor correlation between ACT and APTT (r = 0.76). There were no abrupt closures in the study patients. It was concluded that subthreshold ACTs with high APTTs occur frequently, suggesting the improved suitability of ACT for intraprocedural monitoring of anticoagulation status. If one accepts the minimum amount of anticoagulation for prevention of thrombosis to be that which produces an ACT of greater than 300 s, then an APTT of greater than 90 s does not predict adequate anticoagulation.