ABSTRACT: BACKGROUND: The aim of the present study was to determine the prognostic relevance of thymidine labeling index (TLI) in patients with breast cancer. METHODS: TLI of the primary tumor was measured in 268 patients at the time of the surgical biopsy by an in vitro method. RESULTS: Fifty-four patients had stage I disease, and 138 patients had stage II disease, and 76 patients had stage III disease. One hundred-four patients were found to have low TLI-index (<3%), and 164 patients had high TLI-index (>/=3%). The median follow-up was 71.5 months (range, 6-138 months). The 5-year overall survival (OS) and disease free survival (DFS) rates was 84% and 74%, respectively. Lymph node involvement, tumor size more than 2 cm, high nuclear grade and estrogen receptor negativity were found to be associated with poorer DFS and OS rates. On subgroup analysis, however, the 5-year OS rate was significantly higher in the low TLI-group than in the high TLI-group in patients with stage I disease (100% vs 76%, p = 0.05). CONCLUSION: Our findings suggest that the prognostic significance of TLI appears to be limited to early breast cancer that might help to distinguish patients who need more aggressive adjuvant treatment.