To establish the method of coronary angioplasty by Argon laser, the optimal condition of laser ablation was precisely investigated using bare-ended probe (BEP) and metal tip probe (MTP) of 1.5 mm in diameter. At first, the relationship between laser energy and the depth of the crater on human cadaveric arteries created by laser ablation was examined. Repeated ablation was found to be the most favorite condition and each laser ablation should be restricted between 1 and 5 watts in output and 1 and 2 seconds in ablation time using BEP, and between 5 and 10 watts at 2 seconds using MTP. Secondly, laser ablation was carried out in the canine coronary arteries under beating heart. In this experiment, perforations were seen in some cases using BEP, but no perforation and coronary spasm was observed using MTP. However, temporary drop in distal coronary artery pressure was observed when the size of metal tip was too large for the diameter of the vessel, suggesting vascular thermal constriction. Intraluminal laser thermal ablation using MTP in canine model for establishing the favorite relationship between the size of metal tip and the diameter of the vessel disclosed that MTP/vessel diameter ratio should be 0.7 or less to avoid the vascular thermal constriction. Finally, percutaneous transluminal laser thermal ablations in the coronary arteries were carried out in the living dogs under the condition described above. No perforation, severe arrhythmia, coronary spasm or stenosis were found in this experiment, and it could be concluded that laser angioplasty by Argon is applicable to the treatment of coronary artery disease clinically.