In 50 patients with acute hepatitis serum lipide and lipoproteins were determined at regular intervals and the results were compared with the usual liver function tests. Australia-antigen was established in 26 patients. During the first three weeks of the disease the most striking finding was a significant increase in the triglycerides, which was most pronounced at the end of the second week. Triglyceride levels usually returned to normal during the fourth week. In the course of the disease, lipoprotein electrophoresis showed marked decrease or absence of alpha- and pre-beta-lipoproteins during the first two weeks. During the third week faint alpha and pre-beta bands recurred in most patients. By the end of the fourth week lipoprotein electrophoretic findings were back to normal. There was general correlation between routine tests of liver function and results of lipid analyses throughout the course of the disease. This typical pattern of serum lipid and lipoprotein changes was found with near-consistency in patients with HAA-positive hepatitis. It was also present in the majority of HAA-negative patients, though in these the characteristic discrepancy between hypertriglyceridemia and simultaneous decrease of the pre-beta-lipoprotein band in eletrophoresis was, on the average, absent.