We reported a case of a 22-year old female with a microscopic form of polyarteritis nodosa (PN) who initially manifested Behçet's disease-like symptoms, such as fever, arthralgia, oral aphtha and erythema nodosum, and rapidly progressive glomerulonephritis (RPGN). On admission, her urinalysis showed active nephritic syndrome and her renal function rapidly deteriorated; serum creatinine levels elevated from 1.2 to 3.9 mg/dl within 2 weeks. Skin biopsy specimens from erythema showed panniculitis. Accordingly, she was treated with daily 30 mg of oral prednisolone and three-day intravenous pulse therapy of 1000 mg of methylprednisolone twice. After treatment, skin eruption and oral aphtha disappeared, and the serum creatinine level improved to 1.2 mg/dl. Percutaneous renal biopsy performed on the 28th day showed focal necrotizing glomerulonephritis and hyalinosis of small arteries. Immunofluorescence studies showed only trace stainings for IgG, IgA and beta lc. Electron microscopic findings revealed fusion of the foot process and swelling of endothelial cells, but no dense deposits. Anti-neutrophil cytoplasmic antibody (ANCA) was positive for IgG class with a 40-fold titer by indirect immunofluorescence test and showed a cytoplasmic pattern combined with high urinary IL-8 level (280.1 pg/ml). We diagnosed this case as a microscopic form of PN. ANCA titer and urinary IL-8 correlated positively with the disease activity, and were finally below 8-fold and 58.6 pg/ml, respectively after resolution of RPGN for 42 months. In this case, ANCA was useful not only for differential diagnosis of the patients with systemic vasculitis and crescentic glomerulonephritis, but also for evaluation of the disease activity.