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* BACKGROUND AND OBJECTIVE: Laser sclerectomy may offer advantages to conventional glaucoma filtering surgery by minimizing conjunctival manipulation and subsequent subconjunctival scarring and by providing easier access to difficult locations. It has been theorized that minimizing collateral thermal damage may enhance the success rate and reduce complications associated with laser sclerectomy. The thermal damage induced by the pulsed erbium:yttrium aluminum garnet (EnYAG) laser is notably less than that of other laser modalities, including neodymium:YAG (1.06 pm), ErYSGG (2.79 urn), holmiumYAG (2.10 pm), and holmiumYSGG (2.10 pm). A major obstacle to the clinical use of the ErYAG laser has been the lack of an efficient and reliable delivery system. The single-crystal sapphire optical fiber has an acceptable attenuation rate and favorable characteristics for delivery of the Er: YAG wavelength in a clinical setting. * MATERIALS AND METHODS: An ErYAG laser (2.94 pm) focused into a 300-pm, single-crystal sapphire fiber was used to create ab-externo sclerectomies with varying energy levels and pulse rates in each eye of six anesthetized rabbits and six human cadaver eyes. Specimens then underwent histopathologic analysis and determination of the thermal damage zone. * RESULTS: For the rabbit sclerectomies, there was a significant positive correlation between energy per pulse and the diameter of the thermal damage zone, which averaged 22.0 ± 12.7 urn for all energy levels. For the human sclerectomies, a positive correlation existed between the total energy delivered (mj /pulse X total pulses) and the thermal damage zone, with the mean thermal damage zone being 25.0 ± 9.0 urn. * CONCLUSION: The ErYAG laser with a sapphire optical fiber delivery system is an effective means of creating ab-externo sclerectomies with minimal thermal damage. [Ophthalmic Surg Lasers 1997;28:55-58. J