The efficacy of mammographic screening for older women has not been well studied. The present study was designed to evaluate for downstaging of breast carcinomas associated with mammographic screening in older women. The study population consisted of 130 women age >/=65 years with newly diagnosed breast carcinoma in 1993-1994 and was obtained from women enrolled in a large health maintenance organization. Mammographic screening, if done, was performed by a network of predominantly community-based radiologists. Significant downstaging was found for the breast cancers detected in women who had undergone mammographic screening compared to the breast cancers detected in women who had not undergone mammographic screening. The American Joint Committee on Cancer (AJCC) clinical stage was stage 0 and stage I in 7% (8/107) and 73% (78/107), respectively, of the breast cancers detected in women who had undergone mammographic screening compared to 0% (0/23) and 22% (5/23), respectively, of the breast cancers detected in women who had not undergone mammographic screening (p < 0.0001). Clinical Tis and T1 tumors were found in 7% (8/107) and 74% (79/107), respectively, of the breast cancers detected in women who had undergone mammographic screening compared to 0% (0/23) and 22% (5/23), respectively, of the breast cancers detected in women who had not undergone mammographic screening (p < 0.0001). Of the 102 AJCC clinical stage I-II breast cancers with known pathologic axillary lymph node staging, lymph node stage was N0 for 76% (66/87) of the women who had undergone mammographic screening compared to 53% (8/15) of the women who had not undergone mammographic screening (p = 0.019). The results of downstaging reported in this study are important because earlier staged lesions are associated with an improved prognosis and an increased potential for breast-conservation treatment. These findings have shown that mammographic screening is effective in downstaging breast carcinomas found in older women.