Guinea pigs were inoculated with a reovirus (ReoV) and coronavirus (SARS-CoV) isolated from SARS patients to determine their potential role in the etiology of SARS. Animals infected with ReoV died between day 22 and day 30 postinoculation (PI) while 70% of the animals inoculated with ReoV and SARS-CoV died between day 4 to day 7 PI. The titer of neutralizing antibodies against ReoV and SARS-CoV ranged from 80 to 160 when the animals were inoculated with the two viruses, respectively, while the titer of the antibodies was just below 10 in coinfections. The animal inoculated with ReoV developed diffuse alveolar damage similar to the exudative and leakage inflammation found in SARS patients, and was characterized by diffuse hemorrhage, fibroid exudation, hyaline membrane formation, and type II pneumocytes hyperplasia in alveolar interstitia. The pulmonary epithelial necrosis, excoriation, and early fibrosis of pulmonary tissue were only observed in ReoV-SARS-CoV groups and in SARS-CoV/ReoV groups. Other typical pathological changes included hemorrhagic necrosis in lymph nodes and spleen and hydropic degeneration in the liver. On the contrary, guinea pigs infected with SARS-CoV only developed interstitial pneumonitis. Our experiment demonstrate that ReoV might be one of the primary causes of SARS, since simultaneous coinfection can duplicate the typical pathological changes similar to that of SARS patients. This guinea pig model may provide a useful animal model for SARS.