Adjuvant systemic therapy of breast cancer has been shown to reduce relapses and prolong survival in the women treated. This is true of all subpopulations studied. Six-month multidrug chemotherapy in premenopausal patients, and tamoxifen or short-term chemotherapy in postmenopausal patients, are the treatments of choice in reducing the risk of relapse. In the near future ongoing research may answer some of the open questions surrounding the definition of populations for which the risk of relapse justifies therapy and the optimum manner of using available therapies. The modest but real improvement in prognosis of operable breast cancer was obtained solely by means of clinical trials. Participation in clinical research programs is becoming medically and socially mandatory.