It is not well recognized that monoclonal immunoglobulin heavy chains or their fragments can be amyloidogenic. Amyloidosis due to heavy chains, referred to as AH amyloidosis, is rare with only three cases previously reported. An additional case of AH amyloidosis is reported. To the best of our knowledge, this is the first case of IgM heavy chain amyloidosis. A 59-year-old man presented with nephrotic syndrome. Immunoelectrophoresis detected a monoclonal IgM lambda in his serum and free lambda light chains in his urine. A renal biopsy showed amyloidosis, in which the amyloid deposits stained only for mu heavy chain by immunofluorescent and immunoelectron microscopic studies. This case suggests that monoclonal immunoglobulin heavy chains can be amyloidogenic. Furthermore, this type of amyloidosis shares the same morphologic features with other types of amyloidosis and can involve the kidney to produce nephrotic syndrome.