OBJECTIVES: This study examined the change in heritability of adult body height across birth cohorts in Finland. METHODS: In 1981, cross-sectional questionnaires were completed by 10,968 twin pairs born before 1958. The effect of genetic factors was estimated via genetic modeling. RESULTS: Heritability increased from the cohort born before 1929 (0.76, 95% confidence interval [CI] = 0.65, 0.88 in men; 0.66, 95% CI = 0.55, 0.77 in women) to that born in 1947 through 1957 (0.81, 95% CI = 0.73, 0.87 in men; 0.82, 95% CI = 0.75, 0.89 in women). CONCLUSIONS: Heritability of height increased across Finnish birth cohorts born in the first half of this century and leveled off after World War II. Environmental factors, compared with genetic factors, appear to be more important among women than men.