The purpose of this study is to evaluat.e thallium scanning as a potential test in differentiating malignant from benign lesions of breast. Thirty-one female pat,ients underwent thallium scan of the breast. After intravenous injection of 74-111 MBq(2-3 mCi)of thallium-201, anterior and lat,eral images were obtained. We cornpared thallium scans with pathological results. Of 11 patiens with breat cancers, 10 cases(90.9%) were detected using thallium scan. Thallium scan obtained in one pat,ient who had breast cancer but received several cycles of chemotherapy did not show thallium uptake. The smallest detect,able cancer was 1.5cm in diameter. ln contrast, there is no thallium accumulation in breasts of 17 of 20 patients with benign dis ease(85%), Three cases of 13 fibrocystic disease show thallium uptake in their breast. In conclusion, thallium scan is an effective test in differentiating benign from malignant lesion.