Patient data are presented in Table 1 . The patients had stayed for some time in the hospital or intensive care unit before HSV-1 was isolated, and they had been admitted primarily because of respiratory insufficiency during the course of pneumonia. Patient 4 was admitted into the coronary care unit a few days before intensive care unit admission for cardiogenic pulmonary oedema. All patients had been dependent on mechanical ventilatory support for some time before sampling. They had received adequate antibiotic therapy for pneumonia and had ALI at the time of sampling, which was of otherwise unknown origin. Table 1 shows that patients had radiographic abnormalities but without an increased PLI. Central venous pressure was not elevated, which suggests that the persistent pulmonary infiltrates were not caused by overhydration. In patients 1 and 3 a high-resolution computed tomography scan of the lungs with contrast was obtained; the findings were nonspecific, however, with alveolar consolidations and pleural fluid, even in the presence of interstitial abnormalities with a ground glass appearance in patient 3. In patient 1 a bronchoscopy was performed and there were no mucosal lesions. There was a normal distribution of lymphocyte subtypes in the lavage fluid. A transbronchial biopsy revealed interstitial inflammation with many macrophage deposits, and immunohistochemical staining for HSV-1 was negative. No multinucleated cells or cell inclusions were observed, either in bronchoalveolar lavage fluid from patient 1 or in tracheal aspirates from the other patients. In patients 1-3 concomitant isolation of bacteria by culture was regarded as bacterial colonization. Antibody testing was not done in patients 2-4 but was found to be positive for anti-HSV-1 IgG in patient 1, which is indicative of prior HSV-1 infection.