OBJECTIVE To determine whether there is an increased risk to the corneal endothelium when mitomycin C (MMC) is administered after photorefractive keratectomy (PRK). METHODS Prospective, randomized, double-blind, placebo-controlled crossover trial. METHODS Corneal endothelium was analyzed preoperatively and postoperatively in 18 eyes of nine patients who were administered either MMC- or balanced salt solution (BSS)-supplemented PRK at Codet Aris Vision, Tijuana, Mexico. After laser ablation, one eye was randomly assigned to intraoperative topical MMC 0.02% treatment for 30 seconds, and the fellow eye (the control eye) was treated in a standard fashion with topical BSS. Preoperative pachymetry and endothelial cell count were performed and compared with postoperative measurements after one month and three months. Main outcome measure studied was endothelial cell loss. RESULTS There was no significant difference in the preoperative endothelial cell count between the 2 groups: MMC group 2835 +/- 395, control group 2779 +/- 492, P = .62. In the control group, at one month and three months the difference in the endothelial cell count was not statistically significant (P = .27, P = .14, respectively). However, in the MMC group the endothelial cell loss was statistically significant: at one month 14.7 +/- 5.1%, and at three months 18.2 +/- 9.0% (P = .0006, P = .002, respectively). CONCLUSIONS The use of intraoperative topical MMC 0.02% for 30 seconds after PRK may affect the endothelial cell count.