The role of angioscopic monitoring and aiming for control of laser intervention in the vascular system was initially investigated in 48 vessels in 33 dogs, and the techniques were then applied to 30 patients undergoing intraoperative or percutaneous laserprobe angioplasty treatment for long atherosclerotic occlusions of the femoral and popliteal arteries or well-localized lesions of the superficial femoral artery. Experimental bare argon fiber laser application in 20 normal canine arteries in vivo demonstrated that small-diameter laser fibers could be accurately aimed by manipulations of the scope. However, advancement of the fiber resulted universally in perforation, with extravasation and thermal damage of surrounding tissues after 2 seconds of argon laser energy at low power. In 28 canine and 2 human veins, angioscopically guided metallictipped laserprobes were used to divide 82 valve cusps in preparation for in-situ bypass, with satisfactory aiming and monitoring achieved expeditiously by manipulations of the angioscope. We conclude that angioscopic aiming of lasers is feasible in normal vessels or localized lesions. In contrast, angioscopy has a restricted role for guidance of laser angioplasty in atherosclerotic, occluded arteries, and does not prevent perforation. Postprocedural inspection allows immediate detection of complications and may avert or predict poor outcome.