The prognosis of patients with primary breast cancer depends on prognostic factors per se and on treatment. In certain subgroups of patients, the overall treatment benefits of systemic adjuvant therapy are modest. Therefore, proper selection of patients and treatment modalities is absolutely necessary. Patients with primary breast cancer and a very good prognosis can be classified by known prognostic factors such as tumor size T < or = 1 cm, grade 1, hormone receptor positive tumors, and no other factors indicating poor prognosis. [table: see text] Patients with very poor prognosis are characterized by young age (< 35 years), large tumor size (T > 3 cm), grade 3, hormone receptor-negative tumors, and high S-phase fractions. Tumor size is the best prognostic factor for the decision to treat (breast conservation surgery, primary preoperative or adjuvant systemic therapy) or not to treat. Table 4 shows possible treatment decisions (type and timing of therapy) in primary breast cancer based on known prognostic and predictive variables. The decision to treat or not to treat outside clinical trials should be based on this kind of information. The type and aggressiveness of adjuvant systemic treatment should depend on the receptor status of the tumor and on the age and menopausal status of the patient, as classical predictive factors; however, prognosticators which can describe the metastatic potential of breast cancer cells have yet to be defined.