Study of a disease which affects the lives of so many women in the United States is done most effectively by well-controlled prospective programs. Members of the Cooperative Breast Cancer Study Group have demonstrated their ability to do this, and as an active participating member of this group we have contributed significantly to the knowledge gained in the treatment of this disease. During the next three-year period, it is anticipated that further knowledge will be gained by continuing to pursue carefully controlled studies as we have in the past. These studies will include: (1) Phase I and Phase II studies of new hormonal agents, including newer antiestrogens. (2) Group-wide Phase III studies of effective hormonal programs. (3) Study of chemotherapeutic agents and also evaluation of combination chemotherapy programs. (4) Study of breast cancer tissue for estradiol binding, and urinary hormonal excreting patterns as predictors of hormone responsiveness.