Paraneoplastic cerebellar degeneration (PCD) is a remote effect of cancer mediated by possibly immunological mechanisms. The sera and cerebrospinal fluid of PCD patients containing high titer autoantibody against cerebellar Purkinje cells had been reported. This antibody binds to 62-kD and 34-kD bands of cerebellar Purkinje cell homogenates (anti-Yo antibody). However, it is not always true that the autoantibody of this character on immunoblot and immunohistochemistry recognizes the same molecule. Recently, the DNA sequence encoding the Yo antigen, whose common epitope is a sequence containing leucine-zipper motif, was reported. We made the recombinant protein deduced from the cDNA clone encoding the leucine-zipper motif of the Yo antigen. Using this recombinant protein as the antigen for ELISA, the anti Yo antibodies in the sera and CSF were examined and 3 new patients with PCD possessing anti-Yo antibody were found. The sera of one patient, serially taken during several kinds of treatment were examined with this ELISA system, which revealed that the anti-Yo antibody titer was increased after plasmapheresis and reduced after tumor resection and anti-cancer chemotherapy. The early resection of malignant tumors may prevent the continuous production of high titer anti-Yo antibody and stop the progression of cerebellar tissue damage.