Rheumatoid nodules have been described in many children without evidence of clinical disease. These have been referred to as benign rheumatoid nodules (BRN). Except for one report, no serological abnormalities have been demonstrated. Four children with BRN have been studied. By means of a hemolytic assay, high titers of hidden rheumatoid factor (RF) have been found in the blood, i.e., 19S IgM RF detected after acid separation of IgM-containing fraction from the serum. The median hidden RF titer of the children with BRN was 1:362 and in healthy and disease controls was 1:7. The difference was significant at P less than 0.001. The results indicate that patients with BRN have active complement-fixing 19S IgM RF in their serum. Thus the association of hidden RF with BRN raises the possibility that the deposition of immune complexes (19S IgM-7S IgG) leads to small vessel vasculitis and nodular formation.