OBJECTIVE--To study the effect of anti-CD4 treatment in patients suffering from refractory systemic onset juvenile chronic arthritis (JCA). METHODS--Two children were treated with the mouse CD4 antibody MAX.16H5. The effects on numbers of circulating CD4 T cells, clinical symptoms and C reactive protein (CRP) level were studied and the appearance of human antimouse immunoglobulin antibodies investigated. RESULTS--In patient one, disappearance of fever and malaise and a reduction of arthritic activity were observed together with a reduction in CRP. When disease activity returned eight weeks later, a second successful course of treatment was administered. It was possible to reduce the corticosteroid dosage permanently. In the other child, a first treatment cycle did not alter disease activity. A marked reduction in clinical and laboratory disease activity markers was observed after the second course. Only transient and mild side effects were observed. One patient exhibited a short lasting febrile reaction with chills, the other an urticarial rash. In both patients, human antibodies to mouse immunoglobulin became detectable. The decrease in the number of CD4 T cells in the peripheral blood was only short lasting and numbers returned to normal values within one to eight weeks, even after the second course of antibody treatment and under concomitant immunosuppressive treatment. No sustained clinical remissions could be achieved. CONCLUSIONS--These preliminary observations support the evidence of positive effects of CD4 antibody treatment in refractory systemic onset JCA. Long term efficacy, however, remains to be established.