The acute procedural outcome of percutaneous transluminal coronary angioplasty in 304 patients with unstable angina was retrospectively examined with respect to the influence of prolonged preprocedural intravenous heparin therapy. Clinical and angiographic success in 135 patients receiving heparin therapy for greater than or equal to 24 hours was 91% while such success was noted in 81% of patients not treated with heparin (p = 0.02). The incidence of immediate postprocedural thrombotic vessel occlusion was higher in the nonheparin group than in the heparin-treated group (8.3 vs 1.5%, respectively, p less than 0.01). In addition, the overall rate of thromboembolic target and branch or distal vessel occlusion was 12.4% in the nonheparin group and 1.5% in the heparin-treated group (p less than 0.001). Thus, prolonged preprocedural intravenous heparin administration in this well-defined group of patients with unstable angina resulted in an improved procedural success rate and a significant decrease in the risk of abrupt vessel closure. These observations are concordant with current understanding of the pathophysiology of unstable angina.