Retroperitoneal fibrosis (RPF) with positive antineutrophil cytoplasmic antibodies (ANCA) has been reported in several cases. We herein present the case of a 52-year-old woman who was diagnosed with mediastinal fibrosis (MF) on a thoracoscopic surgical biopsy. The patient had positive myeloperoxidase ANCA and thereafter developed crescentic glomerulonephritis, which was considered to be a form of ANCA-related nephritis. Both the MF and crescentic glomerulonephritis favorably responded to immunosuppressive therapy. These findings suggest a common pathogenesis of these disorders involving ANCA positivity, as reported in patients with RPF.