To describe a case of breast cancer manifested by cerebellar syndrome and to establish a relationship between breast cancer and Paraneoplastic syndromes through the presence of anti- yo antibodies in serum and cerebrospinal fluid of a patient. Our patient was 52 years old, Multipara with 5 children alive. She had been 3 years post-menopausal under Hormonal Replacement Therapy. Weight: 46.7 Kg; Height: 1.60 m; Body Surface Area: 1.59 m(2). Nil history of alcohol or tobacco smoking. Nil history suggestive of malignancies or autoimmune diseases. Her Blood group was oRh positive, nil presence of irregular agglutinins. She was admitted to the neurology service for vertigo and it was determined an isolated cerebellar syndrome. All tests were negative including tumor markers and radiological imaging. The clinical gynecological examination was perfectly normal. The diagnosis hypothesis was "meningo-encephalocerebellitis of viral origin" but with persistence and aggravation of the cerebellar syndrome, despite treatment. We decided to search, antibodies, anti-Hu, anti-Yo, anti-Ri, and anti Ta. Anti Yo was positive + + + in the cerebrospinal fluid and serum of the patient. The search for a gynecological cancer included a mammography which revealed micro calcifications in the left breast + + +. A lumpectomy of the left breast accompanied with x-ray identification of the micro calcifications was done and the histology showed a High Grade Intraductal carcinoma of the left breast with two homes of 3mm and 1 mm, corresponding to an infiltrating carcinoma of the left breast, grade II tumor of Scarff and Bloom (SBRII, 21 N + / 26, RH +, low Ki 67) and Estrogen and progesterone receptor positive +: multifocal cancer. Following the lumpectomy, mastectomy with ganglion clearing was done with adjuvant chemotherapy (FEC 6 Cycles): histology still showed Infiltrating Intraductal Carcinoma of the left breast, grade II tumor of Scarff and Bloom. Radiotherapy was followed and he patient was placed on hormonal therapy with Tamoxifen. The Patient's general condition was good with regression of cerebellar syndrome. Anti-Yo auto antibodies are quasi-specific for gynecological or breast tumors. Several hypotheses have been advanced on the pathophysiology and one wonders if someday, it will fail to do a very early diagnosis of cancer, including the breast cancers on the basis of the antigen-antibody reaction.