The authors performed thermoacoustic computed tomography (CT) with 434-MHz radio waves in five patients with documented breast cancer. Three of the patients underwent imaging before chemotherapy was initiated and two at the conclusion of their primary chemotherapy. In the former three patients, thermoacoustic CT demonstrated contrast enhancement in the region of the tumor. In the latter two patients, no contrast enhancement was seen, and pathologic examination after surgical resection of the area of original tumor confirmed complete remission of disease.