Juvenile chronic arthritis (JCA) was diagnosed in 2 young girls. In one of them, antinuclear antibodies (ANA) were strongly positive during the course of erosive polyarthritis. After 5 years followup, severe renal insufficiency occurred. Antineutrophil cytoplasmic antibodies (ANCA) were positive with a perinuclear pattern on indirect immunofluorescence (IIF) and antimyeloperoxidase (MPO) specificity. Renal biopsy showed severe crescentic glomerulonephritis without significant deposits on IIF. Treatment consisted of prednisone and monthly intravenous cyclophosphamide pulse. Renal failure worsened and hemodialysis was necessary. A 2nd patient was referred for polyarthritis with positive rheumatoid factors without positive ANA. The presence of microscopic hematuria led to the discovery of crescentic glomerulonephritis with positive ANCA of anti-MPO specificity. At latest examination, after prednisone for 10 months and azathioprine for 6 months, the patient had moderate proteinuria with normal renal function. These observations emphasize that in juvenile onset chronic polyarthritis, renal microscopic angiitis with ANCA of anti-MPO specificity may occur.