Sera from 24 children with Henoch-Schönlein purpura (HSP) and 25 controls were tested for rheumatoid factor (RF) of various isotypes. None of the patients with HSP had RF of the IgG or IgM isotypes, whereas one of the controls had IgG RF and one had IgM RF. On the other hand, IgA RF was found in 13 (54%) of the HSP sera, but in only one of the controls (P less than 0.005). The IgA RF showed some cross-species reactivity, but it generally reacted better to human IgG than to rabbit IgG (P less than 0.01) or goat IgG (P less than 0.005). The mean serum IgA concentration was higher in IgA RF positive patients (245 +/- 92 mg/dl) than in IgA RF negative patients (171 +/- 69 mg/dl) (P less than 0.05). Moreover, there was a positive correlation between IgA RF concentration and the level of serum IgA above the mean concentration for age (P less than 0.05). When present, IgA RF concentration tended to be highest during the acute phase of the illness, but there was no correlation between IgA RF and the severity or duration of HSP. These results add to the growing list of immunologic abnormalities in HSP and suggest that IgA RF production may be linked to a basic disturbance of IgA production in HSP.