Nodular regenerative hyperplasia of the liver is an infrequent condition characterized by transformation of the hepatic parenchyma into nodules with only mild fibrosis. Little is known about the etiology except that there is usually an underlying chronic disease, such as Felty's syndrome, which antedates the development of clinical liver disease. It is poorly understood how the associated diseases contribute to the pathogenesis of nodular regenerative hyperplasia. Presented are four cases of nodular regenerative hyperplasia in which macroglobulinemia was also present. This new association suggests to us a hypothesis for the pathogenesis of nodular regenerative hyperplasia. Histologic examination of the livers in these and other cases of nodular regenerative hyperplasia reveals widespread obliteration of the small portal veins. Postmortem angiography of one liver in the present series demonstrated that the nodules were well perfused and that the atrophic areas were poorly perfused with portal blood. This supports the view that atrophy of lobules results from a lack of portal blood supply and that nodules develop from lobules well supplied with portal blood. In each of the clinical conditions associated with nodular regenerative hyperplasia, including macroglobulinemia, inflammatory or thrombotic vascular lesions are found in many organs. Therefore, nodular regenerative hyperplasia may be the hepatic expression of a more widespread vascular disease.