The relationship between breast self-examination (BSE) and pre-treatment clinical stage of breast cancer was assessed in female breast cancer patients consisting of 30 patients practicing BSE monthly, 60 patients practicing occasionally, and 60 patients who had rarely or never practiced BSE. These patients were matched by age, residence, and hospital. More frequent practice of BSE was associated with more favorable clinical stage. The percentage of stage I patients was 33% for monthly performers, and this was significantly higher than the value of 14% for those who had rarely or never practiced. BSE practice was significantly related to tumor size measured before treatment in a dose-response manner. The average values of maximum tumor diameter were 2.5 cm for monthly performers, 3.0 cm for occasional performers, and 3.5 cm for those who had rarely or never practiced. The results suggest that BSE practice increases the probability of detecting smaller cancers at an early stage, and this may lead to a more favorable prognosis.