We isolated rubella virus from lymphoreticular cells in 7 of 19 children with chronic rheumatic disease, including patients with systemic-onset juvenile rheumatoid arthritis (Still's disease) (1 of 5), polyarticular juvenile rheumatoid arthritis (2 of 2), pauciarticular juvenile rheumatoid arthritis (2 of 6), and seronegative spondyloarthritis (2 of 6). In contrast, rubella virus was not isolated from the control group, which included eight normal subjects and eight patients with other connective tissue diseases or traumatic joint effusion. In most members of the study group, mononuclear cells from both synovial fluid and peripheral blood were examined. Rubella virus was isolated from both cell populations in three patients, from only peripheral blood in one, and from only synovial fluid in two. In the children with systemic-onset juvenile rheumatoid arthritis, only peripheral blood was examined, and of the five samples analyzed, one was shown to have rubella virus. Virus was isolated on more than one occasion from four of seven persons. Persistence of rubella virus in lymphoreticular cells in 35 per cent of these cases of juvenile arthritis supports the view that the virus may be an etiologic agent in chronic human joint disease, but further work will be required to support this suggestion.