Warts were found in 25 out of 56 patients with definite or probable systemic lupus erythematosus (SLE) but in only 19 out of 160 control patients. Warts were particularly prevalent in elderly patients with SLE. The corticosteroid and antimalarial drugs used to treat SLE did not influence the frequency of warts. Wart-virus antibodies were found significantly less often in patients with SLE than in controls: antibodies were detected in 23 out of 51 patients and in 40 out of 54 controls. Ihe findings suggest that some deficiency in the immune mechanisms of patients with SLE predisposes them to develop warts. There was an inverse correlation among the patients with SLE between the occurrence of warts and rheumatoid factor activity. This suggests that rheumatoid factor may interfere with resistance to warts.