A retrospective study was conducted at the United States Air-Force Academy (USAFA) to determine the incidence and change in refractive error over a 2.5-year period between the entrance and third academic year physical examinations. For all 994 eyes (497 cadets aged 17 to 21 years) at entrance, there were 37.3% hyperopes, 18.5% emmetropes, and 44.2% myopes of -0.25 D or more, by spherical equivalent (SPEQ). A significant (p less than 0.001) mean SPEQ change of -0.18 D, -0.21 D, and -0.57 D occurred for the hyperopic, emmetropic, and myopic eyes, respectively, over this period. A myopic change of -0.25 D or more was seen in 47.7%, 41.3%, and 74.0% of the hyperopic, emmetropic, and myopic eyes, respectively, with a clinically significant myopic shift of -0.50 D or greater seen in 21.3%, 25.0%, and 55.1% of these refractive error types, respectively. Myopia progression was derived separately from those eyes showing any amount of myopic shift (greater than or equal to -0.12 D), and was -0.42 D, -0.52 D, and -0.75 D for the hyperopic, emmetropic, and myopic eyes, respectively. There was a higher incidence and rate of myopia increase in the higher refractive errors, whether hyperopic or myopic. These findings suggest that 17- to 21-year-olds are not as safe from a myopic change as thought previously, particularly during an intensive educational program.