Idiopathic polymyositis and glomerulonephritis.

We report a 28-year-old male who presented with a clinical picture compatible with idiopathic polymyositis and nephrotic-range proteinuria. Muscle biopsy confirmed the diagnosis of polymyositis and a diagnostic renal biopsy demonstrated IgM mesangial glomerulonephritis. Following a short-course of prednisone, both the myositis and proteinuria resolved. Glomerulonephritis associated with idiopathic polymyositis is rare; however, since it appeared to respond to corticosteroid therapy concomitant with the improvement in the myositis, it was likely an associated immunological complication.