Aims Urethral caruncle is a benign, polypoid urethral mass that occurs almost exclusively in postmenopausal women. Despite that these lesions are routinely managed with topical medications or excision, their pathogenesis is not well understood. We investigated the possibilities of autoimmune, viral and inflammatory myofibroblastic proliferations as possible aetiologies.

Methods In 38 patients with urethral caruncle, we utilised immunohistochemistry for immunoglobulin G (IgG) and IgG4 to assess for a potential autoimmune aetiology. Immunohistochemistry was performed in nine patients for Epstein–Barr virus, BK virus, human herpesvirus 8, human papillomavirus, adenovirus and anaplastic lymphoma kinase.

Results Four patients (11%) showed infiltrates of ≥50 IgG4-positive plasma cells per high power field, of which all showed an IgG4 to IgG ratio greater than 40%. A statistically significant difference (p<0.01) was detected in the mean number of IgG4-positive cells (14.73 per high power field) compared with control benign urethral specimens (mean, 1.19). One patient with increased counts below this threshold had rheumatoid arthritis; none had documented autoimmune pancreatitis or other known manifestations of systemic IgG4-related sclerosing disease. All lesions showed negative reactions for the viral and inflammatory myofibroblastic markers.

Conclusions Urethral caruncle is a benign inflammatory and fibrous polypoid urethral mass of unclear aetiology. It appears unrelated to viral infection and lacks the abnormal expression of anaplastic lymphoma kinase protein, as seen in inflammatory myofibroblastic tumours. Increased numbers of IgG4-positive plasma cells in a subset of lesions raise the possibility that some cases may be related to the autoimmune phenomena of IgG4-associated disease.