A series of 109 like-sexed twin pairs in the age group 30-31 years (54 monozygotic and 55 dizygotic) with one or both members of the twins with myopia was found in the Finnish Twin Cohort. The series was based on a random sample of 1,200 twins in one age stratum of the cohort. The twins received a questionnaire on their health status with special reference to eye diseases and symptoms. The refractive status of the twin pairs was ascertained by asking the twins to send their latest prescription for glasses to the authors or the refraction was obtained from the ophthalmologists or opticians of the twins. The mean difference in refraction between the monozygotic twins was 1.19 dptr in the right eyes and 1.15 dptr in the left eyes. The difference between dizygotic pairs was 2.34 dptr in the right eyes and 2.47 dptr in the left eyes. Analysis of variance showed that the difference in refraction between the two eyes of a twin pair was not significant. The mean difference in refraction between the monozygotic twins was significantly lower than that between dizygotic twins (p less than or equal to 0.001). The intrapair variances were doubled among dizygotic twin pairs compared to monozygotic twin pairs in the male group. The intrapair variance in the female group was 4-fold among dizygotic twin pairs as compared to monozygotic twin pairs. Heritability of myopia was 0.58 (0.74 for males and 0.61 for females) when myopia was considered a dichotomous variable. This result suggests that inheritance has a substantial effect on the etiology of myopia.