Rapid intraoperative scrape cytologic examination for diagnosing surgical margin involvement of specimens obtained by breast conservation surgery was evaluated. Four surgical margins (nipple side, two lateral sides and distal side) of the removed breast tissue were cytologically examined and histologically compared following segmentectomy in 50 breast cancer patients (200 margins). Intraductal carcinoma had a tendency to spread most extensively to the nipple, compared with other margins. The margin positive rate of tumors with ductal spread (DS) of over 20 mm was significantly higher than in tumors with a DS under 20 mm (52.2% vs 7.4%) (P<0.001). Of 50 canditates 10 patients underwent total mastectomy due to positive margins on repeat cytologic examination after re-excision. Four of the 10 patients had an extensive intraductal component on microscopy. The sensitivity, specificity and accuracy of cytology were 96.4%, 90.7% and 91.5%, respectively. Scrape cytology is useful to determine surgical margin involvement after segmentectomy for breast cancer, although overestimation of involvement will tend to result.