Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis and anti-glomerular basement membrane (GBM) disease are two separate diseases, while sometimes they can coexist together. The exact mechanisms are not clear, but due to the rapid progression and poor prognosis, prompt and aggressive treatment is usually required. We treated with steroids combined with cyclophosphamide and rituximab an 84-year-old man with ANCA-associated vasculitis and anti-GBM disease who had prior pulmonary fibrosis and a coexisting anterosuperior mediastinal mass. Conventional therapy including steroids, plasmapheresis and cyclophosphamide failed to attenuate the anti-GBM disease, yet he responded to an alternative treatment of rituximab. This case suggests the efficacy of steroids and immunosuppressant for the treatment of a dual-positive case with an anterosuperior mediastinal mass.