The corneal flap created in LASIK is responsible for most of its advantages in comparison with surface ablation. However, lamellar dissection of the corneal layers in LASIK can also result in serious complications such as corneal ectasia. A 23-year-old man underwent LASIK for correction of -4.75 -2.00@15 in the right eye and -4.50 -2.00@160 in the left eye with a preoperative thinnest corneal thickness of 518 µm/right eye and 513 µm/left eye in 2009. An intended flap thickness and ablation depth in both eyes were 160 µm and 94 µm, respectively, and subsequently, residual stromal bed thickness was 264 µm/right eye and 259 µm/left eye. Several years after surgery, he was referred for the decreased vision. His corrected-distance visual acuity was 0.50 in both eyes. A scissoring reflex was found in retinoscopy. Orbscan imaging was compatible with keratoconus. Anterior segment optical coherence tomography was performed to measure the LASIK flap. It was much thicker (200 µm) than intended (160 µm), and therefore, the residual stromal bed thickness was much thinner. In summary, keratectasia may develop in cases where thicker than expected flaps result in excessive thinning of the residual stromal bed. The obtained results from this case emphasize and remind the importance of intraoperative measurement of flap thickness and using femtosecond and new criteria for patient selection to avoid post-LASIK keratectasia.