Patient with pulmonary fibrosis had delayed final diagnosis of MPA: A 71-year-old male diagnosed with idiopathic pulmonary fibrosis by surgical lung biopsy. A chest CT showed reticular shadows in the basal aspect of each lung. He was treated with 1800 mg/d N-Acetylcysteine. Fourteen months later, his urinalysis revealed hematuria secondary to glomerulonephritis. And he had an intermittent fever for 2 months with an ANCA of 1:40. Percutaneous renal biopsy showed focal segmental necrotizing glomerulonephritis and glomerular crescents. He was diagnosed with MPA and treated with corticosteroids and cyclophosphamide.