• Transscleral cyclophotocoagulation using a free-running, thermal-mode neodymium:YAG laser with a 2.2-mm-diameter sapphire-tipped contact probe was performed on 27 enucleated human eyes to investigate the effect of probe placement and power levels. We concluded from gross, light, and scanning electron microscopic examinations that a tissue destructive effect on ciliary body and epithelium was produced by positioning the anterior edge of the probe tip 0.5 to 1.0 mm from the visible limbus using a 5- to 9-W power setting for 0.7 second. An 11-W power setting resulted in extensive loss of anatomic integrity of the ciliary body. Limbal probe placement produced significant iris and lens damage while positioning the probe 1.5 mm posterior to the limbus affected primarily pars plana. Damage to the sclera was not observed using this method of cyclophotocoagulation.