Paraneoplastic myeloneuropathy has rarely been reported with breast cancer. We report the case of a 59-year-old woman who presented with a peripheral neuropathy and cranial involvement and later developed a myelopathy. The neuropathy was found to be electrophysiologically and histologically demyelinating in nature. Magnetic resonance imaging studies failed to identify any structural brain or spinal cord abnormalities. The patient was diagnosed with breast carcinoma 4 months after initial presentation and underwent resective surgery, radiotherapy, and hormonotherapy. Paraneoplastic antibodies (anti-Hu, anti-Yo, anti-Ri, anti-CV2, anti-Ma, and anti-amphiphysin) were all negative. Her condition did not progress further after cancer treatment. Partial neurologic improvement occurred with oral steroid therapy, with subsequent deterioration on treatment withdrawal.