We report a case of primary position downbeating nystagmus due to an occult breast carcinoma in a 57-year-old woman with progressive oscillopsia and truncal ataxia. Acute nausea and vomiting precipitated hospitalization. Magnetic resonance imaging of the brain was normal, though a sterile mononuclear cerebrospinal fluid pleocytosis was present. Search for an occult malignancy disclosed an adenocarcinoma of the breast. Radical mastectomy and oral corticosteroid therapy did not alter the clinical course of the paraneoplastic syndrome in our patient. Primary position downbeating nystagmus is an uncommon manifestation of an occult malignancy. Our report and review of the literature suggests that investigations necessary for the diagnosis of occult malignancies of the lung, breast, uterus, and ovary be included in the search for cryptic causes of downbeating nystagmus.