PURPOSE. The purpose of the Correction of Myopia Evaluation Trial (COMET) was to evaluate the effect of progressive addi tion lenses (PALs) compared with single vision lenses (SVLs) or the progression of juvenile-onset myopia. METHODS. COMET enrolled 469 children (ages 6-11 years) with myopia between - 1.25 and -4.50 D spherical equivalent. The children were recruited at four colleges of optometry in the United States and were ethnically diverse. They were randomly assigned to receive either PALs with a +2.00 addition (n = 235) or SVLs (n = 234), the conventional spectacle treatment for myopia, and were followed for 3 years. The primary out come measure was progression of myopia, as determined by autorefraction after cycloplegia with 2 drops of 1% tropicamide at each annual visit. The secondary outcome measure was change in axial length of the eyes, as assessed by A-scan ultrasonography. Child-based analyses (i.e., the mean of the two eyes) were used. Results were adjusted for important covariates, by using multiple linear regression. RESULTS. Of the 469 children (mean age at baseline, 9.3 ± 1.3 years), 462 (98.5%) completed the 3-year visit. Mean (±SE) 3-year increases in myopia (spherical equivalent) were - 1.28 ± 0.06 D in the PAL group and - 1.48 ± 0.06 D in the SVL group. The 3-year difference in progression of 0.20 ± 0.08 D between the two groups was statistically significant (P = 0.004). The treatment effect was observed primarily in the first year. The number of prescription changes differed significantly by treatment group only in the first year. At 6 months, 17% of the PAL group versus 30% of the SVL group needed a prescription change (P = 0.0007), and, at 1 year, 43% of the PAL group versus 59% of the SVL group required a prescription change (P = 0.002). Interaction analyses identified a significantly larger treatment effect of PALs in children with lower versus higher baseline accommodative response at near (P = 0.03) and with lower versus higher baseline myopia (P = 0.04). Mean (± SE) increases in the axial length of eyes of children in the PAL and SVL groups, respectively, were: 0.64 ± 0.02 mm and 0.75 ± 0.02 mm, with a statistically significant 3-year mean difference of 0.11 ± 0.03 mm (P = 0.0002). Mean changes in axial length correlated with those in refractive error (r = 0.86 for PAL and 0.89 for SVL). CONCLUSIONS. Use of PALs compared with SVLs slowed the progression of myopia in COMET children by a small, statistically significant amount only during the first year. The size of the treatment effect remained similar and significant for the next 2 years. The results provide some support for the COMET rationale - that is, a role for defocus in progression of myopia. The small magnitude of the effect does not warrant a change in clinical practice.