Heparin anticoagulation is utilized during and after interventional cardiac catheterization procedures to reduce the risk of acute thrombotic coronary artery occlusion. The short half-life of heparin, the importance of maintaining therapeutic anticoagulation, and the time delay inherent in the processing and retrieval of the activated partial thromboplastin time (aPTT) by the hospital laboratory has generated interest in point-of-care heparin monitoring. The activated clotting time (ACT), the aPTT as assessed by both a new portable device, as well as the hospital laboratory, and heparin levels (H) were obtained from the same sample of blood in 100 patients receiving intravenous heparin. There was an excellent correlation between the aPTT determined at the bedside and by the hospital laboratory (r = .89). The ACT did not correlate well with either the laboratory or bedside aPTT (r = .63, .68 respectively). In the sub-therapeutic and therapeutic range, there was essentially no correlation between ACT and H. Only ACT values > 225 sec were predictive of therapeutic or supra-therapeutic aPTTs. ACT values < 225 sec, however, were not useful in predicting degree of anticoagulation. In situations in which the maintenance of therapeutic anticoagulation is critical as well as those in which the determination of lack of anticoagulation is required, the bedside determination of aPTT appears to be a useful tool.