Objective: To ascertain the influence of hormone replacement therapy on clinical and biologic prognostic factors of breast cancer.

Methods: Between 1976-1992, we treated 1081 postmenopausal women for breast cancer at our institution. Of these, 68 were undergoing postmenopausal hormone replacement therapy at the time of diagnosis. These patients were compared with a matched control group of 272 breast cancer patients who had not undergone prior hormone replacement therapy.

Results: Patients who developed breast cancer during hormone replacement therapy had fewer locally advanced cancers (large tumors and extensive lymph node involvement) and more well-differentiated cancers (infiltrating lobular cancers and grade 1 cancer). The number of patients with estradiol or progesterone receptors was lower in the hormone-treated group. Metastasis-free survival curves showed a tendency (P = .05) for better prognosis in hormone-treated patients both overall and in stage T2.

Conclusions: Hormone replacement therapy per se does not affect the prognosis of breast cancer. Regular surveillance during hormone replacement therapy reduces the number of locally advanced cancers and thus improves the survival rate. The higher number of well-differentiated cancers and the distribution of hormone receptivity may reflect interaction between neoplastic tissue and exogenous hormones.