The causes of breast cancer are not fully understood. Although some important risk factors have been identified, many are not easily modifiable (which makes prevention difficult), and it is likely that some risk factors have not been identified. It is known that a virus can cause breast cancer in mice, and previous research by the applicants suggested that a virus; cytomegalovirus, may be important in human breast cancer. This study will test stored blood samples in a group of women who donated blood samples to the Finnish Maternity Cohort, to find out whether patterns of exposure to cytomegalovirus differ for women with breast cancer compared with women who do not have breast cancer. If this research supports a link between cytomegalovirus and breast cancer it could lead to the worldwide prevention of a significant proportion of breast cancer, for instance by immunisation in early childhood.