BACKGROUND Novel approaches are needed for breast cancer patients in whom standard therapy is not effective. 2-Deoxy-2-[18F]fluoro-D-glucose (18F-FDG) was evaluated as a potential radiomolecular therapy agent in breast cancer animal models and, retrospectively, in patients with metastatic breast cancer. METHODS Polyoma middle T antigen (PyMT) and mouse mammary tumor virus-NeuT transgenic mice with tumors 0.5-1 cm in diameter were imaged with 18F-FDG, and tumor to liver ratios (TLRs) were calculated. The radiotoxicity of 18F-FDG administration was determined in healthy mice. PyMT mice with small (0.15-0.17 cm) and large (more than 1 cm) tumors were treated with 2-4 mCi of 18F-FDG, and control C3H/B6 mice with 3 mCi of 18F-FDG. At 10 days after treatment the tumors and control mammary glands were analyzed for the presence of apoptotic and necrotic cells. Five patients with breast cancer and metastatic disease were evaluated and standardized uptake values (SUVs) in tumors, maximum tolerated dose, and the doses to the tumor were calculated. RESULTS Doses up to 5 mCi proved to be non-radiotoxic to normal organs. The 18F-FDG uptake in mouse tumors showed an average TLR of 1.6. The treatment of mice resulted in apoptotic cell death in the small tumors. Cell death through the necrotic pathway was seen in large tumors, and was accompanied by tumor fragmentation and infiltration with leukocytes. Normal mammary tissues were not damaged. A human 18F-FDG dose delivering 200 rad to the red marrow (less than 5% damage) was calculated to be 4.76 Ci for a 70 kg woman, and the dose to the tumors was calculated to be 220, 1100 and 2200 rad for SUVs of 1, 5 and 10, respectively. CONCLUSIONS We have shown that positrons delivered by 18F-FDG to mammary tumors have a tumoricidal effect on cancer cells. The study of breast cancer patients suggests that the tumor and normal organ dosimetry of 18F-FDG makes it suitable for therapy of this malignancy.