Abstract Objective To evaluate the relationship between predicted flap thickness and actual flap thickness and between predicted tissue ablation and actual tissue ablation. Design Prospective, nonrandomized comparative (self-controlled) trial. Participants A total of 60 patients (102 eyes) who underwent laser in situ keratomileusis (LASIK). Main outcome measures Subtraction pachymetry was used to determine actual corneal flap thickness and corneal tissue ablation depth. Other measurements included flap diameter and keratometry readings. Results Actual flap thickness was significantly different ( P < 0.0001) from predicted flap thickness. Fifteen eyes had a predicted flap thickness of 160 μm and a mean actual flap of 105 μm (standard deviation [SD], ±24.3 μm; range, 48–141 μm). Sixty-four had a predicted flap of 180 μm with an actual flap mean of 125 μm (SD, ±18.5 μm; range, 82–155 μm). Seventeen eyes had a predicted flap of 200 μm, with an actual flap mean of 144 μm (SD, ±19.3 μm; range, 108–187 μm). In addition, we found that significantly more tissue ( P < 0.0001) was ablated than predicted. Linear regression of the observed ablation on predicted ablation yielded the following relationship: actual ablation = 14.5 + 1.5 (predicted ablation). Neither flap diameter nor flap thickness were found to increase with respect to steeper corneal curvatures. Conclusions Actual corneal flap thickness was consistently less than predicted regardless of the depth plate used; actual tissue ablation was consistently greater than predicted tissue ablation for the laser used in this study.