A 56-year-old male with chronic lymphocytic leukemia developed extensive erosive mucocutaneous lesions with histologic acantholysis. Immunopathologic studies showed IgG deposition at the intercellular space, C3 deposition at both the intercellular space and the dermo-epidermal junction, and reactivity of the serum to rat urinary bladder epithelium. Autoantibodies in the serum to human epidermal proteins of 210 kD and 190 kD were shown by Western blotting and to proteins of 250 kD, 210 kD, and 190 kD by immunoprecipitation. All these data suggest the diagnosis of paraneoplastic pemphigus. Repeated plasmapheresis resulted in re-epithelialization of the mucocutaneous lesions and reduction in antibody titer from 1:1280 to 1:20. Although this mucocutaneous disease was established as a new autoimmune bullous disease by Anhalt et al. (1990), cases have rarely been reported from Japan. The present patient demonstrates the major characteristics of paraneoplastic pemphigus.