Background: To determine the predictability of excimer laser photorefractive keratectomy (PRK) to correct myopia, astigmatism, or both between -1.00 and -19.00 diopters (D). Setting: Royal Victorian Eye and Ear Hospital, East Melbourne, Australia. Methods: This study comprised 1218 consecutive eyes treated with a VISX Twenty-Twenty excimer laser and followed prospectively for 12 months. Low myopia was treated with one ablation zone (6.0 mm), high myopia with two ablation zones (5.0 and 6.0 mm), and extreme myopia with three ablation zones (4.5, 5.0, and 6.0 mm). Maximum spherical treatment was 15.00 D at the corneal plane. Data were analyzed to determine the predictability of the postoperative outcomes by preoperative refraction. Results: Nine hundred eighty eyes (80.5%) were available for the 12 month follow-up. The predictability of refraction and uncorrected and best corrected visual acuities progressively decreased with increasing myopia, although a comparable percentage of spherical correction was achieved at each diopter of myopia. The likelihood of losing lines of best corrected visual acuity and corneal haze increased with increasing myopia. Conclusion: These data can be used to counsel patients of likely outcomes of excimer laser PRK to correct myopia.