In a retrospective analysis of 199 cases of myeloproliferative diseases a concomitant plasma cell dyscrasia was found in three out of 46 patients with idiopathic myelofibrosis. Chronic myeloid leukemia, polycythemia vera or unclassifiable myeloproliferative disorders were in no case associated with monoclonal gammopathy. One patient with idiopathic myelofibrosis had primarily coexistent IgG-lambda paraproteinemia and increasing osteolytic lesions; histologic evidence of multiple myeloma, however, was insufficient. In the second patient the interval between diagnosis of idiopathic myelofibrosis and IgG-kappa paraproteinemia was 11 years. After a stable period of 9 years' duration the paraprotein level rapidly increased, associated with depression of normal background immunoglobulins and progressive bone marrow failure. The exact nature of this patient's malignant plasma cell dyscrasia remained uncertain. In the third case benign monoclonal gammopathy of the IgM-lambda type was diagnosed 13 years after idiopathic myelofibrosis. A review of the literature confirms a remarkably high incidence of monoclonal gammopathies in idiopathic myelofibrosis. Benign monoclonal gammopathy seems to occur in at least 8% of the patients while only a few cases of concomitant multiple myeloma have been reported. It may be speculated that plasma cell dyscrasias in idiopathic myelofibrosis reflect involvement of the lymphoid lineage in the neoplastic stem cell disorder.