Specific-pathogen-free chickens inoculated with H5N1 highly pathogenic avian influenza (HPAI) viruses isolated in Japan in 2004 were investigated pathologically. The chickens inoculated intravenously with the viruses died within 26 hr after inoculation. Macroscopically, minimal necrosis of the tip of the comb, and hemorrhages of the palpebral conjunctiva, liver, cerebellum, and muscles were rarely observed. Histologically, dead chickens had minimal focal necrosis of hepatocytes with fibrinous thrombi in sinusoids, mild necrosis of splenic ellipsoids with fibrinous exudation, minimal necrosis of the brain, mild necrosis of epidermal cells of the comb with congestion of the lamina propria, and hemorrhages and edema of the lamina propria of the conjunctiva. Virus antigens were seen in the sinusoidal endothelial cells and hepatocytes in the liver, the capillary endothelial cells of the spleen, the capillary endothelial cells and cardiac myocytes in the heart, the capillary endothelial cells and necrotic nerve cells in the brain, the capillary endothelial cells in the lamina propria of the comb, the renal tubular epithelial cells, and the pancreatic acinar cells. The chickens inoculated by natural infectious routes died within 1-4 days after inoculation. Macroscopically, some chickens had hemorrhages in the conjunctiva, edematous swelling of the face and wattles, hydropericardium, hemorrhages of the proventriculus and bursa of Fabricius, increased secretion of tracheal mucus, and congestion and edema of lungs. Histologic lesions by natural infectious routes were similar to those by intravenous inoculation, except for the pancreatic necrosis. This study suggests H5N1 HPAI viruses isolated in Japan in 2004 cause pathologic conditions similar to natural cases.