Among 283 children in a prospective study of arthritis we found 15 patients with a self-limited serum-sickness-like disease consisting of urticaria or joint erythema and mostly polyarticular arthritis. The mean duration of joint symptoms was 5.9 days. A preceding infection was reported in 12 patients and 12 had received drugs, the therapy starting on average 12.8 days before the onset of joint symptoms. In 9 cases the drug was penicillin. Four patients had recurrent attacks. Circulating immune complexes were detected in the serum of 12 patients, but specific IgE antibodies to penicillin only in 3 patients. The estimated annual incidence of the condition was 4.7/100,000 children under age 16.