Objective: To identify the baseline factors independently related to 3-year myopia progression and axial elongation in COMET. Methods: A total of 469 children were enrolled, randomly assigned to progressive addition lenses with a + 2.00 diopter (D) addition or to single vision lenses and observed for 3 years. Eligible children were 6 to 11 years old, with spherical equivalent myopia of -1.25 to -4.50 D, bilaterally. The primary and secondary outcomes, myopia progression by cycloplegic autorefraction and axial elongation by A-scan ultrasonography, were measured annually. Multiple linear regression was used to adjust for covariates, including treatment. Results: Younger baseline age (6-7 vs 11 years, 8 vs 11 years, and 9 vs 11 years, P<.001; 10 vs 11 years, P=.04), female sex (P=.01), and each ethnic group compared with African Americans (Asian, P=.02; Hispanic, P=.002; mixed, P=.002; white, P=.001) were independently associated with faster 3-year progression. Children aged 6 to 7 years had the fastest progression of all age groups, progressing by a mean (±SD) of 1.31 D±0.13 more than children aged 11 years. Females progressed 0.16 D more than the males. Children of mixed, Hispanic, Asian, and white ethnicity progressed more than African American children by 0.49 D±0.16, 0.33 D±0.11, 0.32 D±0.13, 0.27 D ±0.08, respectively. Age and ethnicity, but not sex, were independently associated with axial elongation. Among these myopic children, a 0.5 mm increase in axial length was associated with 1 D of myopia progression. Conclusions: Younger baseline age was the strongest factor independently associated with faster myopic progression and greater axial elongation at 3 years. African American children had less myopic progression and axial elongation than the other ethnic groups. ©2005 American Medical Association. All rights reserved.