A hospital-based retrospective cohort study of benign breast disease (BBD) as a risk factor of future breast cancer (BC) development was conducted. Four hundred and twenty-eight patients with biopsied BBD were followed-up for a median period of 8 years, together with age-matched women with normal breasts (normal control) and BC patients (cancer control), at the ratio of 1:2:2. Twenty-one breast cancers developed, 7 in the cases, 4 in the normal controls, and 10 in the BC controls, showing the relative risk (RR) with 95% confidence intervals (CI) to be 3.5 (1.03-11.9) in the cases with respect to the normal controls. The RR of the cases is not lower than that of contralateral breast cancer incidence. There were no significant differences in the risks of cancers in other organs among the groups. Pathological examination revealed that only atypical hyperplasia increased the RR of BC, as compared with the normal control breast group, or with non-proliferative disease. These results suggest that in a low-risk country, Japan, BBD is a definite risk factor for BC development as in high-risk countries.