The etiology of most rheumatic tissues is unknown; however, recent investigation into this area is rapidly providing new clues to the pathogenesis of many of these diseases. This article reviews several infectious agents (Epstein-Barr virus, parvoviruses, and mycobacteria) and the data supporting their roles in the etiology of rheumatoid arthritis. In addition, current data on the potential roles of Epstein-Barr virus and retroviruses in the etiologies of Sjögren's syndrome and Kawasaki disease are also reviewed. Finally, the reactive arthritides, for which there is stronger evidence of an infectious etiology, are reviewed.