Atypical herpes simplex can mimic a flare of disease activity in patients with pemphigus vulgaris.

We present a 69-year-old white woman with pemphigus vulgaris limited to the oral mucosa who presented with oral pain and difficulty swallowing of 2 days duration, followed by multiple irregular ulcers arising from normal mucosa with no grouping of individual lesions--herpes simplex should be considered in the differential diagnosis of lesions that appear suddenly in patients with PV, particularly if the lesions fail to respond to an increased dose of corticosteroids.