Motor neuron disease has been reported as a rare result of systemic cancers, likely related to an autoimmune effecter mechanism. These patients have been described as having a more rapid onset of symptoms than typical motor neuron disease. Few of these reports demonstrate an association of a cancer-related autoantibody and with motor neuron disease. We present a 54-year-old woman with infiltrating ductal carcinoma of the breast who later developed lower motor neuron disease and was found to have Purkinje cell autoantibodies type 1. The association of lower motor neuron disease as a consequence of breast cancer with this autoantibody profile has not been described previously. This report emphasizes the importance of considering a paraneoplastic syndrome in a patient with an uncommon presentation of motor neuron disease.