Risk of breast cancer subsequent to proven gross cystic disease

3809 women in whom breast cysts were aspirated were followed up to evaluate the observed/expected ratio of subsequent breast cancer. Breast cancer at cyst aspiration was excluded by physical examination and mammography. The first year of follow-up was censored to avoid a prevalence screening effect. Subsequent breast cancers were found either directly or by means of a cancer registry which also provided the expected age and residence specific incidence rates. The number of expected cancers was assessed in person-years (15,915 in the total series). The observed/expected subsequent breast cancer ratio was 1.77 (34/19.15; 95% confidence interval 1.23-2.48, P less than 0.05). The presence of gross cysts was associated with a moderately though significantly increased risk of subsequent breast cancer. Increased surveillance in such patients is not justified.