We compared refractive components, and their changes during a 3-year period, for 79 young adults of whom 29 were youth-onset myopes, 26 were early adult-onset myopes, and 24 were emmetropes. In the initial evaluation we found that mean corneal power was greater for both groups of myopes than for the emmetropes, whereas mean vitreous chamber depth and mean axial length were greater for the youth-onset myopes than for the other two refractive error groups. However, the differences between the two groups of myopes appear to be related to the fact that the mean amount of myopia was significantly greater for the youth-onset myopes than for the early adult-onset myopes. During the 3-year period, mean spherical equivalent refraction for subjects in all three refractive error groups changed in the direction of increasing myopia. For each of the three groups, the only refractive component changes having significant correlations with changes in refraction were vitreous chamber depth and axial length. We interpret these results as indicating that: (1) whether axial elongation occurs before, during, or after the completion of the normal growth period, the result is an eye whose cornea is significantly steeper, whose vitreous chamber depth and axial length are significantly greater, and whose lens differs little from that of an emmetropic eye of a person of the same age and (2) when myopia progresses with time, the progression is due to an increase in axial length that is not fully compensated by a decrease in lens power.(ABSTRACT TRUNCATED AT 250 WORDS)