Pemphigus is a group of autoimmune blistering diseases of the skin and/or mucous membranes caused by the presence of antibodies against adhesion molecules on the cell surface of keratinocytes. In genetically predisposed patients, several factors, including drugs, physical agents, neoplasms, hormones, and viruses, notably herpes simplex virus (HSV), have been hypothesized to trigger or exacerbate the disorder. To clarify whether HSV infection represents an aetiopathogenetic factor for pemphigus or a consequence of the immunosuppressive treatment, skin and/or mucosal swabs from 35 patients with pemphigus vulgaris or pemphigus foliaceus were tested for HSV by polymerase chain reaction. Twenty-three of these patients were newly diagnosed, while the remaining 12 had had a previous diagnosis and were under treatment with low-dosage oral corticosteroids. Repeat swabs were taken two weeks after starting intensive immunosuppressive therapy in 8 HSV-negative patients. All skin swabs (n=27) resulted negative for both HSV-1/2, while oral swabs (n=30) were positive for HSV-1 in 5 out of the 12 patients who were being treated with oral corticosteroids, but in none (n=19) of the non-treated group (p=0.0067, X2 test). Five out of the 8 patients with repeat swabs became positive for HSV-1, prompting us to start antiviral therapy. In conclusion, HSV is unlikely to be a triggering factor for pemphigus, but its presence in pemphigus lesions seems to be a frequent and early complication of immunosuppression.