In this study of 41 patients with progressive juvenile chronic arthritis (JCA), born between 1946 and 1970, it was noticed that 14 were born in the same year (1963). The possibility of a common environmental factor was therefore investigated. Records showed that an epidemic of influenza A H2N2 was present in that year, and the study shows that JCA patients born in 1963 still have a higher level of antibody to influenza A H2N2 than JCA patients born in other years or age-matched controls. This elevation is not seen in a survey of three control viruses. Since this group developed their clinical JCA after the appearance of influenza A H3N2 in 1977, it is suggested that these patients developed a progressive arthropathy because they had been pre-sensitized to influenza A by contact with an earlier strain when in utero.