In Japan, day surgery for breast cancer usually means partial mastectomy without axillary dissection for small carcinoma under local anesthesia in the outpatient clinic. If histopathological examination of the specimens by serial section reveals that the cancer is noninvasive and completely resected with negative surgical margins, no additional surgery under general anesthesia is needed, and the patient does not require hospitalization. We call this procedure "probe lumpectomy". From 1991 to 1998, 169 patients underwent probe lumpectomy in our institution, and there were no major complications. Of these 169 patients, 64 did not require hospitalization. Ipsilateral breast cancer was observed in two patients, and these tumors were diagnosed not as recurrence but as second primary cancers. No distant metastases were observed. As sentinel node biopsy, which is not always easy under local anesthesia, becomes more common, the indications for day surgery or short-stay surgery will expand. As breast cancer is a malignant disease, informed consent and careful follow-up are needed if the treatment is completed only in the outpatient clinic.