The importance of inflammation and immunity on the development of breast cancer is unclear. Breast cancer risk appears to be somewhat lower in women who use nonsteroidal anti-inflammatory drugs (NSAIDs) or who have rheumatoid arthritis (RA). However, breast cancer risk with other systemic autoimmune rheumatic diseases (SARDs) and particularly by estrogen receptor (ER) status is unknown. In 84,778 women in the American population age 67-99 who developed breast cancer and an equal number of women who did not develop breast cancer, we compared the prevalence of RA and other SARDs. The risk of breast cancer, both ER-positive and ER-negative, was significantly reduced 13% among women with RA.  ER-negative, but not ER-positive, breast cancer also was lower among women with systemic lupus erythematosus (SLE). Other SARDs, most of which are treated with NSAIDs, were unrelated to breast cancer risk. This suggests that alterations of immunity with RA, and perhaps with SLE, reduce the risk of breast cancer among older women. However, the reduction in risk is small with RA, so women with SARDs should still adhere to mammographic screening to assure that breast cancers are detected early and treated effectively.