Patients with juvenile rheumatoid arthritis may have an anemia attributable to the chronic disease, to iron deficiency, or to a combination of the two. The contribution of iron deficiency is often difficult to determine by routine laboratory studies. We studied 51 patients with pauciarticular and polyarticular juvenile rheumatoid arthritis with red blood cell counts, indices, free erythrocyte protoporphyrin, and serum ferritin. Fifteen of the 18 who were anemic were restudied after a 3 to 6-month period of iron therapy. Thirteen of the 15 responded by these criteria: a rise in hemoglobin of 1.0 gm/dl or more and an increase in mean corpuscular volume of 3 fl or more; in 11 of these 13, hemoglobin values returned to the normal range for age. These findings indicate that iron deficiency can be a major component of the anemia that is commonly found in patients with active juvenile rheumatoid arthritis.