Three cases of amyloidosis occurring in the later course of idiopathic myelofibrosis were studied at autopsy. In these cases, the amyloid deposition was seen only in the renal glomerulus, which had caused proteinuria. Although their exact nature could not be determined by immunohistochemistry, the amyloid deposits in the three cases were permanganate resistant and exhibited the same organ distribution, indicating a similarity in their character. These cases suggest that amyloidosis might be a complication of idiopathic myelofibrosis, implying that idiopathic myelofibrosis could be a disorder underlying amyloidosis.