A 58-year-old man concurrently developed polymyositis (PM), interstitial lung disease, and nephrotic-range proteinuria. Renal biopsy revealed focal mesangial proliferative glomerulonephritis (mesPGN) with depositions of immunoglobulin and complements. A combination therapy of corticosteroid, intravenous immunoglobulin, and cyclosporine was found very effective for the patient. Glomerulonephritis associated with PM/dermatomyositis (DM) is rare. In our review of related literature, mesPGN was exclusively observed in polymyositis while membranous nephropathy in DM. The mechanism underlying the association between myositis and glomerulonephritis remains to be elucidated.