Intrastromal Corneal Rings (ICRs) have been demonstrated to flatten human corneas when implanted into peripheral intrastromal corneal channels. To study the flattening effect, ICRs of increasing thickness, 0.26, 0.31, 0.36, 0.41, and 0.46 mm, were placed into oversized (approximately 70% depth) intrastromal channels in 38 eye-bank eyes. Each of 33 eyes received one ICR; the mean change in dioptric data was obtained for four meridians using an intraoperative photokeratoscope. Intrastromal corneal rings of increasing thickness resulted in corneal flattening of 3.8 +/- 1.1, 4.9 +/- 0.6, 5.2 +/- 1.1, 5.3 +/- 1.9, and 7.3 +/- 1.6 diopters, respectively, for keratoscope mire 2. One of each size ICR was placed into one of five additional eye-bank eyes; the degree of flattening measured by laser holographic interferometry was 1.8, 2.9, 5.5, 4.7, and 10.1 diopters, respectively, for the central 6 mm corneal zone. These results indicate that the ICR provides a fairly linear flattening relationship over the range of thicknesses tested. Additionally, laser holographic interferometry wave unit maps of preoperative and postoperative corneas demonstrated that the ICR tends to preserve positive corneal asphericity if present preoperatively.