Five patients (two male, three female) mean age 48 years (range 35-81) were studied; only patient No 4 had preexisting medical illnesses (Table 1) and chest x-ray abnormality at disease onset. All the patients presented with a 4-10 days' (median 5 days) history of shortness of breath and flu-like symptoms and rapid clinical deterioration. They were transferred to the ICU for tracheal intubation and ventilation (range 8-25 days; median 17) and diagnosed as having ARF. 15 All the patients received 75 mg twice a day by nasal enteral tube of olsetamivir (range 4-14 days; median 10) and intravenous steroids (range 9-20 days; median 12). After obtaining these results the dose was changed from 75 mg twice a day to 150 mg twice a day through a nasal enteral tube, in accordance with the Brazilian guidelines. The presence of the H1N1 virus was confirmed in all five patients (Table 1) by nasal swab or lung tissue positivity of RT-PCR according to guidelines from the Centers for Disease Control and Prevention. 19 Other microbiological investigations, including the isolation of other viruses, were negative. During the evolution of disease in the patients in the ICU, Staphylococcus aureus was isolated from a blood culture (patients 2 and 3) and Klebsiella spp were identified in tracheal aspirate specimens (patient 1). Patients 1, 2 and 4 are alive, but patients 3 and 5 died of respiratory failure, with concurrent congestive heart failure, hepatic encephalopathy, and acute renal failure. Table 2) . Pulmonary specimens from patients 3 and 5 presented more intense changes at optical microscopy. The membranous and respiratory bronchioles were extensively compromised by epithelial necrosis, squamous metaplasia, and obliteration by fibroplasia ( Figure 1A -F). The parenchyma was modified by extensive alveolar collapse, dilatation of the airspaces, alveolar hemorrhage, and sparse hyaline membrane formation ( Figure 1G -I). There was interstitial thickening, with mild to moderate fibroplasia ( Figure 1I ), but a disproportionately sparse infiltrate of inflammatory cells, mainly histiocytes, including multinucleated forms, lymphocytes and megakaryocytes ( Figure 1J-K) .