Several diseases are known or suspected to be associated with altered levels of hormones and growth factors that may influence breast cancer risk. To elucidate this possibility, we studied the relationship between 23 medical conditions or procedures and breast cancer risk by means of data from a multicentric case-control study conducted between 1991 and 1994 in six Italian areas. The study included 2569 histologically confirmed incident cases of breast cancer (median age 55 years, range 23-74 years) and 2588 control women (median age 56 years, range 20-74 years) admitted to the same hospitals as cases for a variety of acute conditions unrelated to known or suspected risk factors for breast cancer. After allowance for education, parity and body mass index, elevated odds ratios (ORs) emerged for history of diabetes mellitus in post-menopausal women (OR = 1.5, 95% CI 1.1-2.0), hypertension in pregnancy (OR = 1.8, 95% CI 1.0-3.4) and breast nodules (OR = 1.3, 95% CI 1.0-1.7). Risk decreases were associated with ovarian ablation for ovarian cysts (OR = 0.5, 95% CI 0.3-0.7) and with thyroid nodules (OR = 0.7, 95% CI 0.5-0.9) but not with the combination of any type of benign thyroid disease. While most examined conditions seemed unrelated to breast cancer risk, the association with late-onset diabetes is of special interest as it suggests a role of hyperinsulinaemia and insulin resistance in breast cancer promotion. It also points to preventive lifestyle modifications.