Pneumonia caused by herpes simplex virus type 1 (HSV1) is rare and occurs in severely immunosuppressed patients. HSV1 can be detected in bronchoalveolar lavage (BAL) from patients presenting with respiratory failure, but its direct effect on disease is difficult to prove. We demonstrate the causative role of HSV1 in the case of a 44-year-old male with Crohn's disease who presented to the intensive care unit with the acute respiratory distress syndrome after surgery. BAL cells were cultured and immunofluorescence confirmed the presence of HSV1 during the first weeks of illness. Increased IgG titers confirmed the diagnosis of a recurrent HSV1 infection. A lung biposy specimen showed fibroproliferation without pathogens. Immunosuppressive therapy had been stopped and acyclovir was introduced at this time. The diagnostic difficulties in this patient underline the importance of early recognition of viral infection as a potential cause of severe pneumonia in severely ill, immunocompromised patients.