PURPOSE: To determine the usefulness and complications of a newly made metallic tissue marker inserted after mammotome biopsy of breast lesions. MATERIALS AND METHODS: We manufactured a new gold tissue marker and introducer for the insertion after ultrasound-guided mammotome biopsy for breast lesions. The new tissue marker was inserted to 7 dogs and 22 patients. Histopathologic findings were evaluated in all dogs and patients with malignant diseases who underwent surgery. Follow-up mammography and ultrasonography were performed in patients with benign diseases 6 months after the biopsy for the evaluation of complications including migration. RESULTS: In dogs, there was no foreign body reaction in 3 dogs, mild foreign body reactions in 3 dogs, and moderate foreign body reactions in one dog. Meanwhile, the marker was successfully inserted in 17 (77.3%) of 22 patients under ultrasonography-guidance. There were mild foreign body reactions in 4 patients with malignant diseases. On one month follow-up mammography and ultrasonography in 12 of 13 patients with benign diseases with the successful marker insertion showed hematoma with the size ranging from 8 to 23 mm in 8 patients (66.7%). Rotation of the marker without any change of location was seen in 4 patients while rotation with the changed location, about 1cm in distance, was seen in 1 patient. On six month follow-up ultrasongraphy performed in 9 patients showed the disappearance of all hematomas. There was no change of the marker's location on mammography compared to the one-month follow-up mammography. Pain or inflammatory reaction was not noted in any patient. CONCLUSION: This new gold post-mammotome tissue marker showed mild foreign body reaction and no significant complication including migration. It will be very useful in routine practice of mammotome for breast lesions.