The safety of orthokeratological (OK) procedures was assessed by monitoring several ocular characteristics including corneal thickness, refractive astigmatism, corneal astigmatism, correctable spectacle-acuity, corneal edema, corneal staining, and endothelial cell density. Safety was also assessed by reviewing the number and causes of extra clinical visits that occurred because of ocular complications and by determining whether safety factors were related to patients lost to follow-up. Over the 1.2-year period from the baseline examination to the end of the lens-wearing phase of the study, the Treatment (T) and Control (C) groups had, respectively, only small changes in their mean levels of corneal thickness (0.2 vs. 3.2 microns), refractive astigmatism (0.07 vs 0.01 D), and corrected spectacle acuity (0.02 vs 0.018 logarithm of the minimum angle of resolution). Analysis of time trend data obtained from scheduled monthly examinations also showed that changes in mean corneal thickness and mean corneal astigmatism were small and not clinically important in either comparison group during the course of the treatment. As a result of complications from lens wear, the T group required 1.25 times as many extra visits as the C group (p = 0.14). However, none of these visits provided sufficient clinical indication for the discontinuance of lens wear. Loss to follow-up typically occurred because of either poor compliance with the study protocol or loss of motivation caused by minor contact lens-related symptoms such as blurred vision, slight discomfort, or frequently lost lenses. These reasons were similar in both comparison groups. It appears that OK treatment is safe for the types of patients who participated in this study, but it may require more patient monitoring than would be needed to achieve and maintain a physiologically acceptable fit with conventional hard contact lens prescriptions.