A patient with herpes simplex encephalitis presenting as a mass lesion is reported. The brain scan of this patient was unique, resembling a tumor on the lateral projection, and a subdural effusion on the frontal view. The scan pattern described here is unlike that of any previously reported intracranial lesion, and corresponds precisely to the usual localization of pathology in herpes simplex encephalitis. As a consequence, brain scans may be of diagnostic value in the acute phase of this form of viral encephalitis. The persistence of an abnormal pattern for at least 3 months as documented in this report suggests that brain scans also may be useful in the diagnostic evaluation of patients presenting several months after the acute illness.