Higher mammographic density is a strong risk factor for breast cancer. This study was conducted to determine the role of genetic factors on mammographic density measurements in Korean women. Study subjects were 730 women (122 monozygotic (MZ) twin pairs, 28 dizygotic (DZ) twin pairs, and 430 first degree relatives) from the Healthy Twin study. Mammographic density was measured using a computer-assisted method. Pairwise correlations of residual variance of each component of mammographic density were calculated within each pair of twins and family members. Quantitative genetic analysis was completed using SOLAR. Age and measured covariates accounted for 50% of the variation in dense area, 70% of non-dense area, and 67% of percent dense area. Fully adjusted heritability coefficients for dense area, non-dense area, and percent dense area were 0.76 (SE = 0.04), 0.69 (SE = 0.04) and 0.68 (SE = 0.04), respectively. Pairwise correlation coefficients of the adjusted residual variance of the mammographic density measures within MZ pairs and within DZ and sibling pairs combined were, respectively, 0.70 and 0.28 for dense area, 0.52 and 0.31 for non-dense area, and 0.58 and 0.24 for percent dense area. Covariance between dense and non-dense area had a significant genetic basis (correlation coefficient = -0.25, SE = 0.06). The same high heritability of mammographic density in Korean women as found in Western women supports a significant role of genetic determinants in breast cancer development. Genes that are responsible for familial correlation in mammographic density and have opposite effects on dense and non-dense mammographic areas need to be elucidated.