The pathologic changes associated to response to primary chemotherapy in a series of 303 operable breast cancers are evaluated and correlated to patients' follow-up (interval free of disease and survival). In our series, the incidence of microscopic changes related to chemotherapy is 43.9%. Tumor replacement by loose fibrosis is the most common pathologic event. In most cases, the intensity of fibrotic change is proportional to the degree of clinical-mammographic reduction of the tumor mass. However, some discrepancies exist in the sense of absence of microscopic changes in cases of well-documented mammographic reduction as well as in cases without clinical reduction but with large areas of chemotherapy-related fibrosis. The presence of pathologic response is significantly associated with better survival rate.