Paraneoplastic cerebellar degeneration is a rare syndrome that occurs in patients with gynecologic cancer and is characterized by widespread loss of Purkinje cells. To determine whether Purkinje-cell antigens are selectively expressed in the tumors of patients with the syndrome, we examined tumor tissue from 10 patients whose serum contained anti-Purkinje-cell (anti-Yo) antibodies. The origins of the cancers were the breast (five patients), ovary (three), endometrium (one), and fallopian tube (one). We used as controls tumor tissue from 11 patients with ovarian cancer and 10 patients with breast cancer who were neurologically normal. Using immunohistochemical and Western blot analysis, we found that Purkinje-cell antigens were expressed in all the tumors from the 10 patients with paraneoplastic cerebellar degeneration but in none of the tumors from the 21 neurologically normal patients. When IgG from patients with paraneoplastic cerebellar degeneration was affinity-purified to cerebellar Purkinje-cell antigen, immunohistochemical analysis showed that it reacted specifically with the tumor tissue from those patients. We conclude that in patients with paraneoplastic cerebellar degeneration, the anti-Yo antibody results from an immune response to neural antigens expressed by the gynecologic tumors in the patients.