The relation between cigarette smoking and the risk of breast cancer in women was investigated in a population-based case-control study conducted in Adelaide, Australia during 1982-1984. Cases were identified through the South Australian Central Cancer Registry, and, for each case, one age-matched control was selected from the electoral register; in all, 451 case-control pairs were enrolled. Overall, no clear association was found between ever or current smoking and breast cancer risk. In a subgroup analysis, premenopausal ex-smokers were observed to have an increased risk (estimated odds ratio = 2.3). The absence of a protective effect of cigarette smoking on breast cancer risk may reflect the relatively weak estrogen dependence of breast cancer, or the nature of its hormone dependence.