Over a seven-year period, seventy-two patients were diagnosed as suffering from periodic disease; twenty-two of these showed joint involvement. In all patients the disease started in childhood or adolescence. All suffered recurrent attacks of abdominal pain associated with fever. Joint symptoms were caused by a monoarticular arthritis, often with synovial effusion. Recurrent attacks in the same joint were common. Unlike the abdominal attacks, which were of short duration, the joint symptoms were commonly prolonged. Misdiagnoses were common. Seven patients were treated for long periods for atypical rheumatic fever and in two cases the diagnosis was acute septic arthritis. Joint involvement proved transitory and in twenty of the twenty-two cases there was no permanent damage. Biopsy of the synovium showed a non-specific synovitis. It is emphasized that a history of monoarticular arthritis, associated with attacks of fever and abdominal pain, points to the diagnosis. There are no specific laboratory aids and the diagnosis is a clinical one.