Data from 996 newly diagnosed breast cancer patients indicated a highly significant association (P less than 0.001) between periodic breast self-examination (BSE) and pathologic stage of disease. Among women reporting periodic BSE, only small differences were noted between those who practiced monthly and those who practiced several times annually. Average maximum tumor diameter and frequency of tumors 4 cm or large were significantly greater (P less than 0.01) among women who rarely or never practiced BSE. The relationship between the periodic practice of BSE and the diagnosis of breast cancer before nodal involvement was present even after controlling for a wide variety of variables. The regular practice of BSE was associated with a one-third reduction in the likelihood of diagnosis of disease with positive nodes. This difference translated to a 10% decline in five-year mortality for whites and a 17% decline for nonwhites.