PURPOSE: To compare changes in breast parenchymal density among diverse age groups in asymptomatic Korean women with those of Western women, and to evaluate the effect of different patterns of breast parenchymal density on the sensitivity of screening mammography in Korean women. MATERIALS AND METHODS:We analyzed the distribution of breast parenchymal density among diverse age groups in 823 asymptomatic Korean women aged 30 -64 who underwent screening mammography between January and December 1998. On the basis of ACR BI-RADS breast composition, four density patterns were designated: patterns l and 2 related to fatty mammograms, and patterns 3 and 4 to dense mammograms. We compared the results with those for western women. RESULTS: In Korean women, the frequency of dense mammogram was 88.1 % (30 -34 years old), 91.1 % (35 - 39), 78.3 % (40 -44), 61.1 % (45 -49), 30.1 % (50 -54), 21.1 % (55 -59), and 7.0 % (60 -64). Korean women in their 40s thus showed a higher frequency of dense mammograms, but this frequency decreased abruptly be-tween the ages of 40 and 54. In Western women, however, there was little difference between 40 and 54-year-olds: the figures were 47.2 % (40 -44 years), 44.8 %(45 -49), and 44.4 %(50 -54). CONCLUSION: Because the frequency of their dense mammograms shows little change between Western women in their forties and in their fifties, it is clear that between these two age groups, mammographic sensitivity is only slightly different. Because the frequency of dense mammograms is much greater among Korean women in their forties than among Western women og the same age, and among Korean women this frequency decreases abruptly, it appears, however, that the mammographic sensitivity of Korean women is less among those in their forties than among those in their fifties. It is therefore thought that mammography combined with ultrasonography may increase screening sensitivity among Korean women under 50, who have a rela-tively higher incidence of breast cancer in the younger age groups than do Western women.