Ten patients with juvenile rheumatoid arthritis (JRA) developed clinical manifestations of systemic lupus erythematosus (SLE) in 2 1/2 to 21 yr. At onset there was little to distinguish these patients from other children with JRA. Chronic arthritis developed in all and was not different from that seen in JRA. Onset of SLE followed a flare of arthritis in 8 patients, development of serositis in 5, and fever and rash in 5. LE cells and elevated DNA-binding were found in all patients, and glomerulonephritis was demonstrated in the 6 patients on whom renal biopsies were done. This group of patients may represent an important diagnostic subset of children with JRA or SLE.