We investigated the factors influencing the depth of central corneal ablation in LASIK (Laser in situ keratomileusis). Sixty-one patients (122 eyes) underwent LASIK procedure using LADARVISION 4000 (Alcon, Fort Worth, TX, USA). According to the magnitude of difference between the intended thickness (preoperative thickness minus ablated thickness) and the actual postoperative thickness, they were divided in to two groups: group 1 had more than 30 micrometer of difference and group 2 less than 30 micrometer. 40 eyes belonged to group 1 and 82 eyes to group 2. The preoperative spherical equivalent were significantly higher in the group 1 than in the group 2. The central corneal thickness was significantly thinner and the ablation zone diameter was significantly longer in the group 1 than in the group 2. The factors such as preoperative cylinder, central keratometry were not significantly different between both groups. It is concluded that these factors should be considered for improving predictability of the of surgical outcome.