The biologically plausible concept that relief of accommodation may reduce myopic progression, although not confirmed and not universally accepted, has nevertheless strongly influenced the practice of preventive optometry. Yet, no quantitative estimates of the strength of this postulated effect were found in the literature. From a cohort study of 120 young (18 to 20 years of age) male military students with low initial refractive error (less than or equal to +/- 1D, spherical equivalent), estimates were made of the short-term (5 months) effect of two forms of optical treatment to prevent school-related myopia. The cumulative incidence of clinically significant (greater than 0.25 D, spherical equivalent) myopic progression was less [standardized morbidity ratio (SMR) = 0.45] among those prescribed monofocal convex lenses (+1.25 D over far Rx) and prisms (2 delta base-in OU) and even more reduced (SMR = 0.32) among those prescribed bifocals (+1.5 D near adds) compared with those receiving sham treatment (no. 1 pink tint). Placebo-treated patients were more than twice [rate ratio (RR) = 2.2, 1.5] as likely as the monofocal group, and more than three times (RR = 3.2, 1.7) as likely as the bifocal patients, to suffer myopic progression during the period of follow-up. Etiologic fractions (EF, increase in myopic progression if treatment were eliminated) of 37 and 47% and preventive fractions (PF, decrease in myopic progression attributable to the treatment) of 27 and 32% were estimated for the monofocal and bifocal treatments, respectively. These findings suggest that while both modes of therapy are efficacious, the bifocal form is 20% (32/27 = 1.2) more preventive.