Nodular regenerative hyperplasia is defined by hepatocellular nodules distributed throughout the liver in the absence of fibrous septa between the nodules. Most reports have been single cases so that the prevalence and clinical significance of nodular regenerative hyperplasia is uncertain. In this study, the hepatic histology of 2,500 consecutive autopsies was reviewed. A spectrum of nodular transformation was found with nodular regenerative hyperplasia present in 2.6% of autopsy livers and qualitatively similar but lesser degrees of nodular transformation in a further 10.2%. Nodular transformation was also seen in 47% of livers with cirrhosis and 69% with incomplete cirrhosis. Obliteration of many small portal veins was seen in all cases with nodular regenerative hyperplasia, but only 4.7% of these had evidence of portal hypertension. The prevalence of various clinical states was compared in nodular regenerative hyperplasia and in controls. The results confirm, extend and quantify the spectrum of associated diseases. Nodular regenerative hyperplasia occurs in 5.6% of individuals over age 80 and with increased frequency in patients with systemic arteritis, polymyalgia rheumatica, massive tumor infiltration and mineral oil deposition. Nodular regenerative hyperplasia appears to be the hepatic analogue of arterial and arteriolar nephrosclerosis. A new classification of nodular transformation is proposed that encompasses the spectrum of lesions described here and the previously defined entities of focal nodular hyperplasia, partial nodular transformation and "cirrhosis telangiectasia hepatis." The major conclusion is that nodular regenerative hyperplasia is a secondary and nonspecific tissue adaptation to heterogeneous distribution of blood flow and does not represent a specific entity.