Purpose: To characterize the mechanism of injury caused by phacoemulsification and examine the protective effect of ascorbic acid in cultured bovine corneal endothelial cells (CECs). Setting: Goldschleger Eye Research Institute, Sackler Faculty of Medicine, Tel-Aviv University, Tel-Aviv, Israel. Methods: To minimize the direct mechanical effects of phacoemulsification on CECs, cells cultured on gas-permeable flexible membranes were exposed to phacoemulsification. Apoptosis was assayed by immunohistochemistry using anticaspase-3 antibody. Results: Phacoemulsification caused a significantly higher rate of apoptosis (mean apoptotic nuclei per square millimeter, 334 ± 29 [SD] versus 45 ± 12) 48 hours after exposure (P<.001). Results were similar in CECs exposed to hydrogen peroxide for 48 hours as a positive control (mean 345 ± 6 and 376 ± 1 at hydrogen peroxide concentrations of 50 μM and 100 μM, respectively). Addition of ascorbic acid (1 or 10 mM) significantly decreased the 48-hour apoptotic nuclei count induced by phacoemulsification (mean 219 ± 15 and 130 ± 29, respectively) (P<.001). Application of shear forces up to 2000 seconds-1 for 60 minutes did not increase the rate of apoptotic nuclei. Conclusions: Flexible membranes covered with cultured CECs, used as a new model that mimics in vivo conditions, minimized the mechanical damage caused by ultrasonic vibrations and turbulent currents, which destroy cells grown on hard surfaces. Phacoemulsification damage was not mediated by mechanical or shear forces but resulted from free-radical formation that apparently triggered cellular cascades, leading to apoptosis. Cell death was significantly reduced by the addition of ascorbic acid, probably via a free radical-scavenging mechanism.