BACKGROUND: Accumulated exposure to hormones and growth factors during early life may influence the future risk of breast cancer (BC). This study examines the influence of childhood-related, socio-demographic and anthropometric variables on BC risk, overall and by specific pathologic subtypes. METHODS: This is a case-control study where 1539 histologically-confirmed BC cases (23-85 years) and 1621 population controls, frequency matched by age, were recruited in 10 Spanish provinces. Perinatal and childhood-related characteristics were directly surveyed by trained staff. The association with BC risk, globally and according to menopausal status and pathologic subtypes, was evaluated using logistic and multinomial regression models, adjusting for tumor specific risk factors. RESULTS: Birth characteristics were not related with BC risk. However, women with high socioeconomic level at birth presented a decreased BC risk (OR=0.45; 95% CI=0.29-0.70), while those whose mothers were aged over 39 years at their birth showed an almost significant excess risk of hormone receptor positive tumors (HR+) (OR=1.35; 95% CI=0.99-1.84). Women who were taller than their girl mates before puberty showed increased postmenopausal BC risk (OR=1.26; 95% CI=1.03-1.54) and increased HR+ BC risk (OR=1.26; 95% CI=1.04-1.52). Regarding prepubertal weight, while those women who were thinner than average showed higher postmenopausal BC risk (OR=1.46; 95% CI=1.20-1.78), associated with HR+ tumors (OR=1.34; 95% CI=1.12-1.61) and with triple negative tumors (OR=1.56; 95% CI=1.03-2.35), those who were heavier than average presented lower premenopausal BC risk (OR=0.64; 95% CI=0.46-0.90) and lower risk of epidermal growth factor receptor positive tumors (OR=0.61; 95% CI=0.40-0.93). CONCLUSION: These data reflect the importance of hormones and growth factors in the early stages of life, when the mammary gland is in development and therefore more vulnerable to proliferative stimuli.