OBJECTIVE The fraction of cells that incorporate 3H-thymidine (3H-dT labeling index [3H-dT LI]) proved to be a prognostic indicator in patients with node-negative and node-positive resectable breast cancers treated with locoregional treatment alone or with adjuvant combination chemotherapy. In this study, we assessed the prognostic role of 3H-dT LI alone and in association with other pathologic and biologic variables in a series of 249 women with node-positive breast cancers treated with adjuvant endocrine therapy. METHODS All patients were postmenopausal, had resectable estrogen receptor-positive (ER+) tumors, and had received tamoxifen for at least 1 year after radical or conservative surgery plus radiotherapy. The median follow-up duration was 48 months. RESULTS The 4-year relapse-free survival (RFS) rates were significantly lower for patients with large tumors (> 2 cm), with more than three positive lymph nodes, with low (< 150 fmol/mg proteins) ER content, without progesterone receptors (PgRs), or with rapidly proliferating tumors. 3H-dT LI provided prognostic information independent of axillary node involvement, ER content, PgR status, and tumor size, with an estimated odds ratio (OR) higher than that of tumor size, lymph node involvement, or ER concentration. In addition, 3H-dT LI and PgR in association were able to identify patients with different risks of relapse within subsets of tumors with less or more than three positive nodes. CONCLUSIONS 3H-dT LI provides prognostic information complementary to PgR, tumor size, lymph node involvement, and ER content in the prediction of RFS of postmenopausal patients with node-positive, ER + resectable tumors treated with adjuvant hormone therapy.