We have developed a new, steerable microangioscope for performing percutaneous coronary angioscopy. The angioscopic catheter is made of polyethylene, contains a distal balloon for occlusion of blood flow during imaging, fits through an 8 Fr percutaneous coronary angioplasty (PTCA) guiding catheter and is steerable by means of a guidewire. Initial feasibility was demonstrated in animal studies involving both peripheral and coronary arteries. Angioscopic findings in normal vessels included normal endothelial surface, collateral blood flow, and side branches. Angioscopy was also performed after balloon denudation of the arteries, laser-assisted balloon angioplasty, and stent implantation. In early human clinical trials we have performed percutaneous angioscopy in six patients undergoing percutaneous coronary angioplasty without complications. Findings in these patients include atherosclerotic plaque, thrombus, and dissection. This new device has great potential for use in the research, diagnosis, and treatment of coronary atherosclerosis.