Renal involvement of mixed connective tissue disease (MCTD) shows systemic lupus erythematosus (SLE)-like immune complex glomerulonephritis. The prognosis of this condition is generally good. We report the case of an elderly female patient with MCTD who developed autoimmune pleurisy and rapidly progressive glomerulonephritis. Myeloperoxidase antineutrophil cytoplasmic antibody (MPO-ANCA) was positive with a titer of 59.0 EU. Anti-DNA antibody and complement levels were normal. Renal biopsy revealed crescentic glomerulonephritis and mild mesangial proliferation. However, immunofluorescence examination revealed immune-complex glomerulonephritis. These findings suggest that the renal involvement of MCTD developed concurrently with MPO-ANCA-related glomerulonephritis.