The relationship between brassière size, as an indicator of breast size, and breast cancer risk was considered in a case-control study conducted between 1991 and 1994 in six Italian centres. Cases were 2,557 women, below age 75, with histologically confirmed breast cancer, and controls were 2,566 women admitted to hospital for a wide spectrum of acute, non-neoplastic, non-hormone-related diseases. Odds ratios (ORs) of breast cancer and their 95% confidence intervals (CIs) were obtained from multiple logistic regression equations including terms for study centre and age, as well as main breast cancer risk factors. A slight inverse relationship was observed between breast size and the risk of breast cancer, with an OR of 1.37 (95% CI 1.05-1.80) for the smallest brassière size compared with the largest; the increase in risk disappeared after adjustment for main recognized breast cancer risk factors, with an OR of 1.16 (95% CI 0.87-1.54) for brassiere size ≤ 1 compared with ≥ 5. No significant heterogeneity in risk of breast cancer with breast size was found in strata of age at diagnosis, parity, age at first birth, age at menopause, family history of breast cancer, benign breast disease, ever use of oral contraceptives and/or hormone replacement therapy. Thus, this study, based on a large number of caucasian women, provides conclusive evidence of a lack of appreciable association between breast size and breast cancer risk in this Italian population.