Objective: To determine whether ectasia after laser in situ keratomileusis (LASIK) is related to residual corneal bed thickness. Design: Retrospective noncomparative case series. Participants: Thirty-two eyes of 16 patients with refractive errors of -4.00 to -18.00 diopters were examined. Intervention: LASIK was performed. The topography of the posterior corneal surface was examined with the Orbscan slit scanning corneal topography/pachymetry system. Main Outcome Measures: The difference in the elevation of posterior corneal surface regarding the best-fit sphere was measured. Results: After surgery, mean bulge of 17.2±7.2μm was found in eyes with residual corneal bed of 250μm or greater, whereas 41.0±22.1μm was seen in eyes with bed thickness less than 250μm (t=4.29; P=0.000). Conclusion: Posterior corneal bulge is correlated with the residual corneal bed thickness. The risk of ectasia may be increased if the residual corneal bed is thinner than 250μm.