The relation between cigarette smoking and breast cancer risk was investigated in a multicentric case-control study conducted in Italy on 2,569 women with incident, histologically confirmed breast cancer and 2,588 control women admitted to hospital for acute, non-neoplastic, non-hormonal, non-gynaecological or smoking-related conditions. Compared with women who had never smoked, current smokers had a multivariate odds ratio (OR) of 0.84 [95% confidence interval (CI) 0.7-1.0] and former smokers an OR of 1.14 (5% CI 0.9-1.4), while the OR for ever vs never smokers was 0.93 (95% CI 0.8-1.1). The ORs were 1.02 for < 5 cigarettes per day, 0.99 for 5-14 cigarettes per day, 0.78 for 15-24 cigarettes per day and 1.18 for > or = 25 cigarettes per day. No consistent pattern of risk was observed according to duration of smoking, age at starting and time since starting smoking. Compared with never smokers, former smokers had ORs of 1.45 for < 3 years since stopping smoking, 1.79 for 3-6 years, 1.16 for 7-15 years and 0.74 for > or = 16 years. No heterogeneity emerged across strata of selected covariates. Thus, this study, one of the largest conducted in Europe to date, does not support the presence of any association of practical importance between cigarette smoking and breast cancer risk.