OUTLINE: This is a randomized study. Patients are stratified by participating institution, when and whether they receive adjuvant chemotherapy, axillary lymph node status, and hormone receptor status. All patients are randomized within 12 weeks of definitive surgery. Patients receiving adjuvant chemotherapy prior to protocol treatment are randomized within 6 weeks after the last dose of chemotherapy. One group of patients receives daily oral tamoxifen, while a second group receives daily oral tamoxifen plus octreotide (long-acting release formulation) by monthly depot injection. Treatment in both groups continues for 5 years or until disease recurrence or development of a second malignancy. Patients are followed monthly for 4 months, every 4 months for 3 years, and every 6 months thereafter.