Using an automated method for detecting mammographic mass, the authors evaluated the relation between quantitatively measured density and the risk of breast cancer in a case-control study among Japanese women. The case subjects were 146 women newly diagnosed and histologically confirmed with breast cancer at a general hospital. A total of 659 control women were selected from those who attended a breast cancer mass screening at this hospital. Significantly increased odds ratios (ORs) of breast cancer were observed for breast densities of 25-49 and 50-74%, but not for densities of 75-100% as compared with 0% in premenopausal women after controlling for covariates (ORs = 4.0, 4.3, and 1.4, respectively). In postmenopausal women, ORs were significantly increased for breast densities of 25-50% (OR = 3.0) and 50-100% (OR = 4.2). Total breast area was significantly associated with the risk of breast cancer independent of density percent or dense area in postmenopausal women. These data suggested that mammographic density was associated with the risk of breast cancer in Japanese women as is the case in Caucasian women. However, the associations of the risk of breast cancer with breast size and a high breast density greater than 75%, needs to be confirmed in future studies.