The paraneoplastic neurological disorders provide perhaps the best known example of naturally occurring tumor immunity in humans. For example, patients with paraneoplastic cerebellar degeneration (PCD) appear to suppress the growth of occult breast or ovarian tumors that express a neuronal antigen termed cdr2. PCD patients harbor cdr2-specific CTLs in their peripheral blood, and these cells are likely mediators of the tumor suppression. Whereas cdr2 therefore appears to be the target of an effective immune response in patients with PCD, the general relevance to cancer patients has been unclear, due in part to reports indicating that cdr2 is not expressed in tumors obtained from neurologically normal patients. We have reexamined this question, and we find that cdr2 is widely expressed in such tumors, indicating that cdr2 is in fact an important tumor antigen in the general population of breast and ovarian cancer patients.