Seven out of 22 children with monarticular juvenile rheumatoid arthritis (MJRA) developed involvement of other joints between six months and three and a half years from the onset. In the other 15 patients the disease has remained monarticular for between one and 16 years (mean six years). Chronic iridocyclitis was seen in three of the five boys, two with antinuclear antibodies. Children with MJRA and antinuclear antibodies should have periodic ophthalmic assessment. Synovial biopsy was of value primarily in excluding other cases of arthritis, but there was only limited correlation between the histological findings and the subsequent course of the disease.