Objective: To determine whether eye color changes after 6 years of age.

Design: Longitudinal data on eye color were obtained from the Louisville Twin Study, Louisville, Ky. Twins (n = 1513 [individuals]) were assessed at least once and most twins (n = 1386) were examined on 2 or more occasions. Parents of twins were also examined at the study inception, 128 of whom were assessed again from February 1989 to October 1993.

Main outcome measure: Eye color was assessed at each examination by matching the iridial coloration of the subject to 1 of 15 painted glass eye anterior segments, similar to those in artificial eyes, mounted on a circular disk. The spectrum ranged from light blue (1) to dark brown (15).

Results: Among whites (n = 1359), the eye color of 3.8% to 8.6% of the sample twins became 2 U or more darker or 2 U or more lighter during 3- to 9-year intervals between 6 years of age and adulthood (> 18 years, < 24 years). Among identical (monozygotic) twin pairs, there was a high degree of concordance in eye color (r = 0.98 [P < .001]), while in fraternal (dizygotic) twin pairs, the concordance was less pronounced (r = 0.49) and decreased with age (r = 0.07). Among the sample of the mothers of twins, 9% had irides that lightened by 2 U or more during the follow-up period.

Conclusion: Most individuals achieve stable eye color by 6 years of age. However, a subpopulation of 10% to 15% of white subjects have changes in eye color throughout adolescence and adulthood in the eye color range that can be expected to reflect changes in iridial melanin content or distribution. These data also suggest that such changes in eye color, or the propensity to such changes, may be genetically determined.