Clinical, light and electron microscopic studies of 6 cases of acute and 8 cases of chronic active hepatitis were made and results were compared. Light microscopically acute viral hepatitis was dominated by intralobular changes characterized by ballooning degeneration, random individual and small group cell necrosis of hepatocytes with mononuclear cell reaction, cholestasis and Kupffer cell proliferation, while chronic. active hepatitis was dominated by periportal and portal changes characterized by piecemeal necrosis, heavy mononuclear cell infiltration, moderate fibrosis and mild biliary proliferation. Kupffer cell proliferation with large amount of diastase-resistant PAS positive pigments and patchy reticulin condensation were noted in both acute and chronic active hepatitis, but reticulin condensation was more advanced in chronic active hepatitis. Electron microscopically, acute hepatitis showed marked changes of nucleus, RER, bile canaliculus, and decrease of glycogen content, while chronic active hepatitis showed marked changes of mitochondria with giant fomrs and intramitochondrial inclusion, increase of polyribosomes and glycogen content, and appearance of collagen bundles in the sinusoidal wall. Kupffer cell changes were very marked in both acute and chronic active hepatitis showing large numbers of dense bodies. These dense bodies in acute cases were in the form of secondary lysosomes while they were residual bodies in chronic cases. A case which showed ground glass appearing cytoplasm by light microscopy showed massive fibrillar and tubular structures by electron microscopy. In all cases, no definite virus-like particles were observed within either the nucleus or cytoplasm. From the data, it was evident that distinction between acute and chronic active hepatitis is more clearly made with light microscopy, and the ultrastructural changes of intralobular lesions showed more similarities than differences. The meaning of minor ultrastructural differences is not clear and further evaluation is desirable. Clinically, acute cases showed higher serum bilirubin, transaminase level and hypoalbuminemia while in chronic active hepatitis serum globulin level was higher and hepatomegaly was more regularly observed.