A case of paraneoplastic pemphigus (PNP) associated with Castleman's disease in the retroperitoneal region is reported. A 54-year-old woman presented at our hospital because of multiple regions of severe oral mucositis. Biopsy of the buccal mucosa was performed, and the histopathological diagnosis was a lichenoid lesion. The results of indirect immunofluorescence and immunoblotting strongly suggested PNP. General screening by CT scanning, endscopy of the upper and lower gastrointestinal tracts, and nuclear medical examinations was performed to search for the cause of PNP. A 60×43mm mass was found in the right retroperitoneal region on CT. The tumor was extirpated by urologists at our university hospital. The histopathological diagnosis of the mass was Castleman's disease. The oral lesion was diagnosed as PNP and was treated with high-dose prednisolone and cyclosporine A gargle. After treatment, the oral lesion resolved completely. The clinical course of the patient has been good, with no evidence of recurrence 1 year after discharge from our hospital.
