The prognostic relevance of 3H-thymidine-labeling index (3H-TdR-LI) was retrospectively evaluated in 523 women with resectable node-positive breast cancer given adjuvant combination chemotherapy consisting of cyclophosphamide, methotrexate, and fluorouracil (CMF) +/- Adriamycin (doxorubicin; Farmitalia, Carlo Ezba, Italy). The 5-year relapse-free survival (RFS) and overall survival (OS) rates were significantly higher for patients with slowly proliferating tumors (3H-TdR-LI less than or equal to 2.8%) compared with rapidly proliferating tumors (RFS, 66% v 50%, P = .0007; OS, 85% v 73%, P = .0012). In the analysis of RFS, 3H-TdR-LI provided prognostic information independent of axillary node involvement, tumor size, and estrogen receptor (ER) status, with an estimated lower hazard ratio compared with the degree of nodal involvement, but equivalent to that of the other indicators. Conversely, nodal involvement was found to interact with 3H-TdR-LI and receptors on survival. Present findings confirm that tumor cell kinetics represents a prognostic indicator also in an adjuvant situation. 3H-TdR-LI can substantially contribute to a more precise definition of high-risk patients within subsets having the traditionally favorable prognostic characteristics.