BACKGROUND: A standard intra-operative procedure for assessing sentinel lymph node metastasis in breast cancer patients has not yet been established. PATIENTS AND METHODS: One hundred and thirty-eight patients with stage I breast cancer who underwent sentinel node biopsy using both imprint cytology and frozen section were analyzed. RESULTS: Seventeen of the 138 patients had sentinel node involvement. Results of imprint cytology included nine false negative cases (sensitivity, 47.1%). In contrast, only two cases of false negatives were found on frozen section (sensitivity, 88.2%). There were two false positive cases identified by imprint cytology (specificity, 98.3%). On the other hand, frozen section had 100% specificity. CONCLUSION: These findings suggest that frozen section is superior to imprint cytology for the intra-operative determination of sentinel lymph node metastasis in stage I breast cancer patients.