We report a case of a persistent penile plaque on the glans penis of allegedly more than 20 years' duration, which was refractory to circumcision and local treatment. Over the years, the patient repeatedly presented with a circumscribed inflammatory lesion of the glans penis, diagnosed as Zoon's balanitis on the basis of clinical aspects and two biopsies. Because of unresponsiveness of the lesion to circumcision and focal steroid infiltration, repeated biopsies were performed in an attempt to rule out malignancy. Two further biopsies were carried out. One again showed the features of a plasmacellular inflammation, while the other finally revealed the histopathologic features of erythroplasia of Queyrat (carcinoma in situ or Bowen's disease of the glans penis). We assume that either the former biopsy specimens were taken from a plasma cell-rich reactive infiltrate around the neoplastic lesion, or that carcinoma in situ may have arisen due to the chronic inflammation of Zoon's balanitis plasmacellularis. Radiotherapy was performed with good clinical response and subsequent histopathologic proof of complete remission of the lesion.