<i>Objective:</i> To identify through a substudy of a larger, multicenter study of adjuvant treatment in primary operable breast cancer patients any possible correlation between cellular proliferation rate, measured by thymidine labeling index (TLI), and perioperative chemotherapy (periCT). <i>Methods:</i> TLI was measured in slices of early breast carcinoma patients. The main trial was designed to randomize patients after primary surgery to receive one cycle of periCT consisting of cyclophosphamide, epidoxorubicin and 5-fluorouracil, or no periCT. <i>Results:</i> Of 600 patients randomized into the main study, 197 were eligible for inclusion in this substudy. Characteristics of patients were quite similar to those of the entire population entered into the main study. The TLI cutoff value in our series was 0.7% expressed as the median percentage ratio of thymidine-labeled cells undergoing DNA synthesis in the tumor cell population of specimens from the 197 patients. No differences were observed in terms of relapse-free survival (RFS) and overall survival (OS) after grouping the patients by TLI value (low and high) and by treatment. Among node-negative patients, a significant improvement in terms of OS (p = 0.02), but not RFS (p = 0.06), was seen in patients with a high-TLI value who underwent periCT versus controls. <i>Conclusions:</i> TLI may be a useful tool for the identification of node-negative patients with high-TLI values who may benefit from periCT.