Exemestane or Anastrozole in Treating Postmenopausal Women Who Have Undergone Surgery for Primary Breast Cancer

RATIONALE: Estrogen can stimulate the growth of breast cancer cells. Hormone therapy, using exemestane or anastrozole, may fight breast cancer by reducing the production of estrogen. It is not yet known whether exemestane is more effective than anastrozole in preventing the recurrence of breast cancer. PURPOSE: This randomized phase III trial is studying exemestane to see how well it works compared to anastrozole in preventing cancer recurrence in postmenopausal women who have undergone surgery for primary breast cancer.