A nationwide cohort with risk stratification according to risk factors for breast cancer was examined for the association between hormone replacement therapy (HRT) and breast cancer. Data from 4,558,376 postmenopausal women who underwent breast cancer screening and regular health checkups from 2009 to 2014 were assessed using the Korean National Health Insurance Service database. Current or previous HRT use was reported in a total of 696,084 (15.3%) women. During a median follow-up of 5.35 years, 26,797 (0.6%) women were newly diagnosed with breast cancer. Observations confirm the correlation between HRT use and breast cancer risk. Proportional increase in the risk was observed with duration of HRT; the risk was identified differing according to body weight and breast density. Women who were leaner and those who had dense breasts were noted to be at higher HRT-related risk of breast cancer.