PURPOSE: To determine the risk factors for undercorrection of myopia after photorefractive keratectomy and to evaluate the efficacy and safety of retreatment. METHODS: A VISX Twenty/Twenty excimer laser was used to treat myopia and myopic astigmatism of up to -15.00 diopters (spherical equivalent) at the corneal plane. Retreatments were performed primarily for undercorrection with or without coexistent corneal haze or abnormalities on videokeratoscopy. Not every patient who was undercorrected requested retreatment. RESULTS: Of 645 eyes (440 patients) followed up for more than 12 months, 58 eyes (9%) required retreatment. The retreatment rate increased with increasing myopia: 17 (5%) eyes with myopia of less than -5.00 diopters, 30 (13%) eyes with myopia of -5.01 to -10.00 diopters, and 11 (19%) eyes with myopia of greater than -10.00 diopters required a second procedure. A higher retreatment rate was observed after astigmatic corrections than after spherical corrections. Corneal haze after retreatment was no greater than that observed after initial procedures. Twelve months after primary procedures, 378 (75%) of 504 eyes were within 1.00 diopter of emmetropia, whereas 27 (69%) of 39 eyes attained this result after retreatment. A total of 383 (76%) of 504 eyes attained uncorrected visual acuity of 20/40 or better after primary procedures, compared with 25 (64%) of 39 eyes after retreatment. CONCLUSIONS: The risk of undercorrection requiring retreatment after photorefractive keratectomy increases with the magnitude of the primary treatment. Although less successful than initial procedures, retreatment appears to enhance the results of photorefractive keratectomy.