Nodular regenerative hyperplasia of the liver developed in five patients who had the toxic oil syndrome caused by ingestion of adulterated cooking oil. This hepatic complication was detected from 19 to 37 months (mean-2.5 years) after the onset of the toxic oil syndrome. Nodular regenerative hyperplasia was asymptomatic, although all patients had persistently abnormal liver function. Hepatomegaly was present in four cases, mild jaundice in three and signs of portal hypertension in two. Pathogenesis of nodular regenerative hyperplasia in toxic oil syndrome is unknown, but probably microcirculatory disturbances within the liver might have played a role. Fibrosis in Zone 3, sinusoidal dilatation and occasionally intralobular hemorrhage were seen in three cases; in one of them, characteristic lesions of veno-occlusive disease were also present. In another case, endarteritis changes of hepatic arterioles were evident.