Purpose: Evaluation of ultrasound-guided percutaneous marking of breast cancer using an "O-Twist Marker" (BARD GmbH, Karlsruhe, Germany) in patients undergoing neoadjuvant chemotherapy. In the event of complete remission, the lesion of interest can be missed after completion of therapy. In these cases marking of the tumor site is essential. Materials and Methods: We monitored patients with breast cancer proven by biopsy who were undergoing neoadjuvant chemotherapy with ultrasound. In cases in which the follow-up ultrasound examination after the second chemotherapy cycle showed a significant tumor mass reduction (> 30%), we pinpointed the lesion with an ultrasound-guided "O-Twist Marker". The position of the marker was documented by ultrasound and mammography. Between January and November 2006, we marked 7 patients. Results: The visibility of the marker was excellent on mammography, but only moderate on ultrasound. The marker did not cause any imaging interference. The instrument was easy to operate. We did not detect any migration of the marker. In 3/7 patients it would have been possible to locate the remaining tumor tissue after the completion of neoadjuvant chemotherapy without the marking. In 4/7 cases the clip was very useful or even essential for preoperative site determination. Conclusion: The use of "O-Twist Marker" is a reliable and effective technique for locating breast cancer sites in remission during neoadjuvant chemotherapy. The visibility of the marker on ultrasound should be improved. In 57% of the cases tumor marking was advantageous for surgical procedures.