Twenty children with systemic juvenile rheumatoid arthritis, aged 0.9-13.7 years, were studied with regard to their serum ferritin concentration at diagnosis and during follow-up, ranging from 2 to 9 years. At diagnosis, during fever, the concentration was extremely high. The median value was 935 micrograms/l. The values were unrelated to other manifestations of the disease or laboratory findings. During glucocorticoid treatment, the serum ferritin concentrations normalized rapidly, usually within a few weeks. The rate of normalization reflected the response of the fever to treatment. Later, subnormal concentrations were found, which were unrelated to the activity of the arthritis. Thus, serum ferritin is a useful guide when tapering glucocorticoid dosage.