Six autopsy cases of the rare, diffuse type of hepatocellular carcinoma (HCC), as classified gross anatomically according to the strict definition, have been studied. The prominent clinical feature was the rapid deterioration of the patient's general condition, terminating in hepatic failure. The liver size enlarged quickly, at a perceptible speed, often accompanied by abdominal pain. Diagnosis of this particular type of HCC was difficult, and celiac angiography and scintiscan of the liver were only suggestive when considered together with other laboratory data. Hepatitis B surface antigen was positive in all three patients in whom it was tested. The entire liver was studded with minute, uniformly sized tumor nodules, evenly distributed throughout. Some of them were grossly indistinguishable from cirrhotic nodules. All livers had an underlying cirrhosis which was characterized by relatively small regenerative nodules with thin stromas. Large portal branches at the hilum contained tumor thrombi in all patients, except for one case in which left lobectomy was followed by intraportal dissemination. Histologically, all tumor nodules represented intrahepatic metastases via the portal vein system. Tumor cells were poorly differentiated. These findings suggest that the diffuse type of HCC most frequently, if not always, represents intrahepatic, widespread portal metastases which have occurred within a short period of time.