A 42-year-old woman with locally advanced breast cancer developed headache just after completing adjuvant chemotherapy. Magnetic resonance imaging revealed a mass located in the left subthalamic nucleus (STN) and involving the posterior part of the thalamus and the hypothalamus. The patient refused a radiologically guided biopsy and gamma knife treatment was not financially possible. Palliative whole brain radiotherapy with hormonal therapy was administered. The patient gained 19 kg body weight during 4 months follow up because of hyperphagia. This solitary tumor, either a breast cancer metastasis or a primary tumor, involving the STN is extremely unusual.