To investigate the clinicopathologic characteristics of severe acute respiratory syndrome (SARS). Three autopsy cases were studied retrospectively. Routine HE stain was used to study all the cases. Part of the lung tissue specimens were studied further with Macchiavello's stain, viral inclusion body stain, reticulin and PAS stains, immunohistochemistry, thin sections with staining, light microscopy and transmission electronic microscope investigation. The earliest symptom of all 3 cases was hyperpyrexia and followed by progressive dyspnea and appearance of lung field shadows in X rays findings. Pulmonary lesions included: bilateral and extensive consolidation, localized hemorrhage and necrosis, desquamative alveolitis and bronchitis, alveolar proliferation and desquamation, accumulation of protein exudates, mononuclear cells, lymphocytes, and plasma cells as well as hyaline membrane formation in alveoli and viral inclusion bodies were seen in the alveolus epithelial cells. The exudated organization tended to become glomeruloid organizing pneumonitis in a few avaoli. Lesions of the immune organs included: large patchy necrosis in the spleens and localized necrosis in the lymph nodes were seen. Bone marrow became restrained. There were lesions of systemic small vasculitis including edema of the perivascular tissue and vascular wall of the small veins with localized fibrinoid necrosis distributing in the heart, lungs, kidneys, adrenal glands and the striated muscles accompanying with mononuclear cells and lymphocytes infiltration. Thrombosis was seen in part of the small veins. In addition, there were also the systemic poisonous changes including: degeneration and necrosis of the parenchyma cells in lungs, liver, kidneys, heart and adrenals. Electronic microscopy demonstrated clusters of virus particles seen in the lung tissue. SARS is a systemic disease. Lungs, immune system and systemic small vessels are the main target organs attacked by the virus. Extensive consolidation of lungs, formation of hyaline membrane to a large extent, respiratory distress and decrease of immune function are the main causes of death.