The Prospective Evaluation of Radial Keratotomy (PERK) study is a nine-center clinical trial of a standardized technique of radial keratotomy in 435 patients who had simple myopia with a preoperative refractive error between -2.00 and -8.00 diopters (D). The authors report results for one eye of each patient. The surgical technique consisted of eight incisions using a diamond micrometer knife with the blade length determined by intraoperative ultrasonic pachymetry and the diameter of the central clear zone determined by the preoperative refractive error. At 3 years after surgery, 58% of eyes had refractive error within 1.00 D of emmetropia; 26% were undercorrected and 16% were overcorrected by more than 1.00 D. Uncorrected visual acuity was 20/40 or better in 76% of eyes. The operation was more effective in eyes with a preoperative refractive error between -2.00 and -4.37 D. Between 1 and 3 years after surgery, the refractive error changed by 1.00 D or more in 12% of eyes, indicating a lack of stability in some eyes. In the 435 eyes, there was a small number of complications including six eyes that lost two or three lines of best-corrected acuity, 16 that experienced vascularization of the incisions, 2 that had delayed bacterial keratitis, and 4 that had recurrent epithelial erosions.