PURPOSE: To report a case of an abnormally thick flap that was detected with the use of anterior segment optical coherence tomography (OCT) prior to planned LASIK retreatment surgery. METHODS: A 43-year-old man presented 6 years after bilateral LASIK surgery with uncorrected visual acuity (UCVA) of 20/20 in the right eye and 20/100 in the left eye, and best spectacle-corrected visual acuity (BSCVA) of 20/20 in the right eye and 20/20 in the left eye (refraction -2.25 -1.25 x 131). Anterior segment OCT was performed before LASIK retreatment surgery in the left eye, RESULTS: The flap was found to be 394 μm and the residual stromal bed was too thin (152 μm) to allow for safe LASIK retreatment surgery. After waiting 4 months to ensure refractive stability, photorefractive keratectomy was performed. Postoperative UCVA was 20/20 piano. CONCLUSIONS: The use of anterior segment OCT in the preoperative examination for planned LASIK retreatment surgery provided more reliable data.