The knowledge and practice of breast self-examination (BSE) was investigated among 500 women with operable breast tumors between 35 and 64 years of age (all successively operated) and 652 healthy women, matched with the previous group for 5-year age groups from 35 to 54 years. Only 39.9% of breast cancer women and 34.5% of the controls practiced BSE, starting from 45 years, and it is used less frequency by the women with a poor education and those in a lower economic bracket. Among the 500 breast cancer patients, those who practiced the self-examination had a higher number of tumors with a diameter no greater than 2 cm and a lower number with a diameter larger than 4 cm. In addition, in this group the percentage of unaffected axillary lymph nodes (N-) was 58.8% compared to 48.8% for the group that did not practice self-examination, and the percentage of the cases with more than 3 metastatic lymph nodes (N+ greater than 3) was 20.0% in the first group and 27.3% of the second one. These differences were statistically significant. The correlation between tumor diameter and the histologic lymph node stage is equally evident, since there is a progressive reduction in N- cases and a contemporary progressive increase in N+ (greater than 3) cases with increasing breast tumor diameter. Therefore, we can infer that the lack of practice of BSE causes a diagnostic and therapeutic delay, which is responsible for aggravation of the prognosis.