We present a case of proliferative glomerulonephritis with peculiar IgM deposition associated with sarcoidosis. A 62-year-old woman, who had been diagnosed with sarcoidosis 3 years previously because of abnormalities on chest X-ray radiophotographs and lymph node pathology, was admitted to our hospital for the evaluation of proteinuria and microscopic hematuria. Laboratory findings showed renal dysfunction (creatinine clearance, 52 ml/min), a moderate range of urinary protein (1.51 g/day), and increased serum lysozymes (20.7 microg/ml; normal range, 3.4-8.6 microg/ml). Serum calcium level was within the normal range. Renal biopsy revealed immune complex glomerulonephritis (IgM deposition type) with a membranoproliferative pattern, without granuloma or calcium deposition. Corticosteroid (initial dose of prednisolone [PSL], 1 mg/kg per day) was administered, but neither renal function nor urinary protein improved. She then became nephrotic and her renal function gradually deteriorated. To our knowledge, among uncommon glomerulonephritides with sarcoidosis, five cases of immune complex glomerulonephritis with IgM deposition have been reported. Immune complex glomerulonephritis with IgM deposition is unusual and could be related to sarcoidosis; it may be a characteristic pathology which could provide a clue to elucidate the pathogenesis of sarcoidosis.