Cigarette smoking has been hypothesized to have both carcinogenic and anti-estrogenic effects that may offset each other to produce no overall effect on breast cancer risk. A full-term pregnancy also appears to have opposing effects on breast cancer risk: 1) an adverse effect shortly after delivery and 2) a beneficial effect over time. If the transient increase risk of breast cancer is due to the growth-enhancing consequences of elevated pregnancy hormones on already initiated cells, then cigarette smoking during pregnancy, through its anti-estrogenic effect, can be expected to dampen this risk. Conversely, with its carcinogenic effect, cigarette smoking during pregnancy might also reduce the long-term protection against breast cancer afforded by a full-term pregnancy. We propose to examine the effects of cigarette smoking on the risk of postpartum breast cancer occurring at different intervals following delivery. We will use a database that links together the Swedish Medical Birth Register, National Cancer Register, and Register of Causes of Death. Members of the study population are all mothers who delivered a liveborn or stillborn baby after a gestation period of at least 28 weeks in Sweden between 1973 and 1998. We have adopted a nested case-control sampling design to allow more efficient analyses. Cases are approximately 3,500 women who had one or more childbirths between 1973 and 1998 and who had a breast cancer diagnosis during the same period. For each case subject, five controls who were born in the same year as the index case, were alive at the date of the diagnosis for the index case, and had not been diagnosed with breast cancer by that date, will be randomly selected from the source population. Logistic regression analysis will be applied to examine cigarette smoking as a risk determinant for postpartum breast cancer adjusting for age, parity, and age at first full-term pregnancy.