The relationship between various body size indices and breast cancer risk before and after menopause was elucidated by means of a case-control study conducted between June 1991 and April 1994 in 6 Italian centers on 2,569 patients aged below 75 with histologically confirmed breast cancer, and on 2,588 controls admitted to the hospital for a wide spectrum of acute, non-neoplastic, non-hormone-related diseases. Weight and, more consistently, body mass index (BMI, kg/m2) at diagnosis were inversely related to pre-menopausal breast cancer risk and directly to post-menopausal risk. An 8-unit increase in BMI resulted in an odds ratio of 0.8 for pre-menopausal and of 1.2 (significant) for post-menopausal women. Risk seemed to increase gradually after menopause in the 7th (OR for an 8-unit BMI increase, 1.3) and 8th decades (OR, 1.6) of life. Conversely, height, waist-to-hip ratio, bra cup size and weight (or BMI) in adolescence and in young adulthood did not exert a significant or consistent influence on breast cancer risk. The apparent relationship with BMI at middle age and weight gain between age 30 years and diagnosis was eliminated by allowance for BMI at diagnosis. The age-related pattern of the association between BMI and breast cancer risk after menopause may reflect a duration-risk relationship, and resembles the effect of post-menopausal estrogen use, which seems greater among older women.