In this article, we demonstrated the unexpected reduction of optical zone diameter in myopic keratomileusis and its causes. Mathematical evaluation of optical zone diameter was made possible through a module added to a computer program. Optical zone diameter was reduced more in smaller corrections than in higher corrections. Practical measurements were parallel to mathematical expectations except in some small corrections between -4.00 and -6.00 diopters, where it was even more reduced than calculated. We were able to enlarge the optical zone diameter using the same computer module. Refract Corneal Surg 1991:7:161-1 66.)