The ablation of atherosclerotic lesions without collateral thermal or shock wave damage is thought to be a key element for successful laser angioplasty. This study evaluated the effectiveness of pulsed ho1mium:YAG laser (2.1 pm wavelength) for this application. Fresh normal tissue (n = 139) and arteriosclerotic canine arteries (n = 21) as well as formalin-preserved normal canine (n = 31) and atherosclerotic human arteries (n = 177) were irradiated under saline via a 600 pm diameter fiber placed perpendicular to the intimal surface with 0-10 gm of force. The laser was operated in the free running mode (FRM; 250 psec pulsewidth, 5 Hz, 30-7,100 mJ/mm2) and in the Q-switched mode (QSM; 200 nsec pulsewidth, 6 Hz, 30-1,100 mJ/mm2). Following the experiments, the samples were prepared for histologic and morphometric analysis. Ablation thresholds in the FRM were 60 and 180 mJ/mm2 in fresh and preserved canine tissue, respectively. Ablation thresholds in the QSM for fresh and preserved canine tissues were 75 and 180 mJ/mm2, respectively. Thresholds for human atherosclerotic tissue were dependent on the amount of calcification. In the QSM and FRM, there were no samples that could not be penetrated at 1,100 mJ/mm2 and above. Histologic examination of the FRM samples revealed confined columns of tissue ablation, with approximately 55-250 pm and 70-140 pm zones of thermal effect being apparent in the fresh and formalinpreserved samples, respectively. The QSM samples (fresh and preserved) revealed minimal thermal effect (0-20 pm) to the intima and media (smooth muscle cells) between 300 and 1,100 mJ/ mm2, whereas thermal effect in the adventitia (collagen) was 64- 172 pm with the same energy fluence. We conclude 1) that the thermal effect can be optimized by shortening the pulsewidth (Q switching at 200 nsec) or by limiting the total energy delivered and 2) that controlled ablation of both normal and atherosclerotic vascular tissue can be accomplished with the ho1mium:YAG laser.