Microscopic polyangiitis, a condition recently differentiated from macroscopic periarteritis nodosa, is characterized by small vessel damage, pauciimmune necrotizing glomerulonephritis and presence of perinuclear anti-neutrophil cytoplasmic antibody (p-ANCA). Arthralgia is a common symptom often present early in the disease, and other joint manifestations have also been reported. We report a case with polyarthritis as the first manifestation. Perinuclear ANCA was found in a moderate titer. A renal biopsy done six months after the onset of joint symptoms to investigate rapidly progressive renal failure established the diagnosis. The p-ANCA exhibited antimyeloperoxidase specificity. In the discussion we review the diagnosis of microscopic polyangiitis and of concomitant polyarthritis and p-ANCA production. ANCA is present in some patients with rheumatoid arthritis or systemic lupus erythematosus. It is important to determine the specificity of the ANCA since presence of p-ANCA with antimyeloperoxidase specificity in a patient with polyarthritis is highly suggestive of systemic vasculitis.