Paraneoplastic pemphigus is a rare autoimmune disease characterized by polymorphous mucocutaneous lesions in association with underlying neoplasia. We report two Chinese patients, one with chronic lymphocytic leukemia and the other with thymoma. Both of them initially presented with severe oral erosion with lichenoid features that resisted all the regular therapy. The routine histology was not typical in both patients, with features of liquefaction degeneration of the basal cell layer and subbasilar lymphocytic infiltration. Important differential diagnoses for the two cases include lichenoid eruption secondary to chemotherapy, pemphigus, and pemphigoid. Immunohistologic testing played an essential role in the diagnoses. Direct immunofluorescence shows both intercellular and basement membrane deposition of immunoreactants. Indirect immunofluorescence using rat bladder as the substrate has been shown to be an inexpensive and reliable diagnostic tool. Our patients' clinical and immunopathologic features were consistent with a diagnosis of paraneoplastic pemphigus, a special subset of pemphigus. Patients' conditions were improved after high-dose systemic corticosteroid therapy after underlying malignancies were treated. Keywords: diagnosis, direct immunofluorescence, indirect immunofluorescence, oral presentation, paraneoplastic pemphigus