AIM In node-negative breast cancer patients, several factors for survival have been evaluated and currently, some of them are accepted for their prognostic and/or predictive values after validation in the separate data sets. The prognostic significance of increases in the number of pathologically detectable axillary lymph nodes in the node-negative patients could not been established clearly. To address this question, we have reviewed our patients' records. METHODS A retrospective cohort study was conducted in pathologically node-negative patients who underwent modified radical mastectomy for stage I and II breast cancer. Survival and multivariate prognostic factor analyses were carried out to determine whether the number of tumour-free lymph nodes in complete axillary dissection material in addition to known factors was significant for the outcomes. RESULTS Two hundred and seventy consecutive patients were eligible to enter the trial. The median observation time and the median number of tumour-free lymph nodes were 61 (from 30 to 120) months and 18 (from 10 to 44), respectively. The cohort was divided into the groups according to the number of nodes. The 5-year event-free and overall survivals were 92.5 and 98.3% for patients who had 18 lymph nodes or less, and 70 and 86.7% for those who had more than 18 negative nodes, respectively (P < 0.00001). Multivariate analysis for event-free survival demonstrated that the number of lymph nodes (Relative risk: 3.2 and 95% confidence interval: 1.7 to 5.9) in addition to the pathological tumour size and age was the most important independent prognosticator. In similar, multivariate analysis for overall survival showed that the number of lymph nodes together with the tumour size was the significant indicator (RR of cancer-specific dying in patients who had more than 18 nodes: 3.1 and 95% CI: 1.2 to 8.5). CONCLUSION The increases in number of tumour-free lymph nodes are clinically important and this parameter should be taken into consideration in the breast cancer patients without metastatic lymph nodes.