Tigers and birds infected with H5N1 highly pathogenic avian influenza virus in Thailand in 2004 and 2005 were investigated histologically and immunohistochemically. Histologically, tigers had interstitial pneumonia and hepatocytic necrosis, and a native chicken had focal necrosis of the parenchyma of brain. Immunohistochemically, influenza virus antigens were demonstrated in the necrotic foci of the liver in tigers. Influenza virus antigens were detected in the necrotic area of brain, necrosis of the spleen, and vascular endothelium of the whole body in the native chicken and open-billed storks.