Introduction The incidence of breast cancer is continuously increasing in Japan. The early life exposures such as being breastfed in infancy have been hypothesised to influence subsequent breast cancer risk.

Methods We investigated the relationship between having been breastfed and breast cancer risk in a hospital-based case-control study of women aged 30 and over (573 breast cancer cases and 2155 cancer-free controls). Data on reproductive factors, lifestyle, and history of having been breastfed were collected using a self-administered questionnaire.

Results and Conclusion After adjustment for known risk factors of breast cancer, no association for having been breastfed was observed overall (OR 1.20; 95% CI 0.82 to 1.75). Analysis stratified according to birth year (<1950, ≥1950) demonstrated heterogeneity in the association of having been breastfed with breast cancer risk between the two birth-year groups (p for interaction =0.0006); having been breastfed was associated with a decreased risk among women who were born before 1950 (OR 0.59, 95% CI 0.35 to 0.99), whereas it was associated with an increased risk among women born after 1950 (OR 1.65, 95% CI 0.91 to 2.98). These findings indicate that early nutrition has some effect on breast cancer risk. In Japan, the use of standard formula supplement began to spread around 1950. Endocrine disrupters such as organochlorines were also introduced to the food chain at around the same time. The heterogeneity of breast cancer risk between the two birth-year groups may therefore be attributable to these environmental changes related to infant feeding.