Primary tumor size, an essential clinical parameter, is assessed by oncologist during the physical examination of a patient with breast cancer. The results of such assessment constitutes a basis for classification of breast cancer staging, selection of proper primary treatment, selection of type of surgical treatment and initial evaluation of patient prognosis.The aim of the study was to describe changes in biological characteristics of breast cancers relative to increases in primary tumor size.Material and methods. All women treated surgically for breast cancer in the Department of Surgical Oncology, Medical University in Łódź, from 1999 to 2004, were included in our study. We confined our analysis to primary operable, invasive, primary tumors less than 3 cm. 639 cases comprised the study group. We analyzed the associations between primary tumor size and other routinely assessed clinical and pathological parameters.Results. We found statistical associations between primary tumor size and the presence of metastases in axillary lymph nodes (p<0.0001), presence of extracapsular extension of lymph nodes metastases (p<0.0001), presence of tumor cells emboli (p<0.0001), absence of expression of estrogen receptor (p=0043) and absence of progesterone receptor (p=0.0196) in cancer cells. No association was found between tumor size and histologic tumor type (p=0.205). In a subgroup of ductal carcinomas, tumor size was associated with the grade of cancer (p=0.0002).Conclusions. Increases in tumor size are accompanied by increasing incidence of unfavorable prognostic factors and decreasing incidence of favorable prognostic factors. Tumor size constitutes the source of direct and indirect prognostic information.