A 20-year-old woman, who was suffering from appetite loss, weight loss and livedo reticularis for one and half months, was referred to our hospital. On admission, laboratory studies demonstrated proteinuria (1.0 g/g Cr), hematuria (erythrocytes': 50 - 99/HPF), ,.enal dysfunction (Cr : 2.09 mg/dL), elevated C reactive protein (CRP: 10.82 mg/dL), elevated MPO-ANCA titer (11.6 U/mL) and elevated pentraxin3 (PTX3: 24.05 ng/mL). Her kidney and skin biopsy revealed massive crescentic necrotizing glomerulonephritis and leukocytoclastic vasculitis, respectively. She was diagnosed with microscopic polyangiitis (MPA), and treated with 500 mg/day of intravenous methyl-prednisolone (mPSL) for 3 days followed by 40 mg/day of oral PSL, rituximab (375 mg/m² once a week for a month) and plasma exchange. When PSL tapered to 30 mg/day in 4 weeks, her renal function was only partially recovered, while the CRP level had been normalized and the MPO-ANCA titer was almost negative (3.6 IU/mL). To evaluate histological activity, a second renal biopsy was conducted, which showed fibrocellular crescents in 32% of her glomeruli. The PTX3 level remained high (14.82 ng/mL) at that point. Taken together, the vasculitis was considered to be active still. Steroid pulse therapy for 3 days was administered again, followed by oral PSL 30 mg/day. Her renal function completely recovered in 70 days. The PTX3 level also normalized in 161 days. PTX3 is one of the short pentraxins, produced by a variety of cell types in response to pro-inflammatory signals such as IL-1 and TNF-α. It was reported that PTX3 reflects activity of vasculitis independently from CRP. In the presenting case, when the second renal biopsy revealed a histologically active lesion of the vasculitis, PTX3 was elevated independently from CRP and MPO-ANCA, suggesting that PTX3 may be a more sensitive marker of the disease activity than other tests.