An early, acute arthritis was observed in 8 of 22 mice experimentally infected with respiratory syncytial virus. No abnormalities were found in the joints of the 12 control mice. The pathological changes in joints of the infected animals included pannus formation and erosion, resembling changes that occur in arthritis in man, including rheumatoid arthritis. It is proposed, therefore, that viruses may play an aetiological role in acute and chronic arthritis in man. These investigations provide a model for studying pathology of arthritis in mice produced by a virus which also infects man.