Purpose.To report the acute effects of laser in situ keratomileusis (LASIK) on the corneal endothelium.Methods.Twenty eyes of 10 consecutive patients (mean age, 38.1 ± 10.84 years) underwent bilateral simultaneous LASIK for myopic astigmatism (spherical equivalent ranging from −1.75 to −7.13 diopters) without any complications. Each eye was evaluated by slit-lamp biomicroscopy and noncontact specular microscopy preoperatively, within 15 minutes after LASIK and 1 day after surgery. Specular microscopy images were then analyzed to calculate endothelial cell density (ECD), coefficient of variation (CV) of cell size, and percentage of hexagonal cells.Results.All corneas demonstrated marked alterations in endothelial cell morphology by slit-lamp biomicroscopy within 15 minutes after surgery that resolved by the first postoperative day. Central corneal endothelial analysis by noncontact specular microscopy confirmed pleomorphism with definite loss of hexagonality. Mean ECD was calculated to be 2,816.3 ± 286.02 cells/mm2preoperatively, 2,750.85 ± 327.95 cells/mm2on day 0 (p= 0.395), and 2,810.55 ± 218.48 cells/mm2on day 1 (p= 0.461). Mean CV was 32.65 ± 7.29 preoperatively, 34.4 ± 6.19 on day 0 (p= 0.412), and 30.9 ± 5.54 on day 1 (p= 0.067). Mean percentage of hexagonal cells was 63.35 ± 10.76 preoperatively, 47.55 ± 9.69 on day 0 (p= 0.000009), and 60 ± 9.3 on day 1 (p= 0.00003).Conclusion.Qualitative and quantitative changes in endothelial cell morphology (i.e., decreased endothelial cell hexagonality) demonstrate that LASIK does induce an acute effect on the corneal endothelium that may represent transient endothelial cell edema.