We attempted to ascertain the mechanism of portal hypertension and ascites complicating acute hepatitis in 66 patients who underwent transvenous liver biopsy and measurement of hepatic venous pressure gradient. Increase in hepatic venous pressure gradient was related to the severity of acute hepatitis, as indicated by the significant correlation between the values for hepatic venous pressure gradient and serum bilirubin, serum albumin or coagulation factor V, and by its higher value in patients with, than in patients without, encephalopathy. Hepatic venous pressure gradient was higher in patients with, than in patients without, ascites (12.5 +/- 3.4 vs. 8.4 +/- 3.6 mmHg, respectively; p less than 0.001). No ascites was clinically detectable in the patients in whom hepatic venous pressure gradient was below 6 mmHg. We tested the hypothesis that sinusoidal collapse due to liver cell dropout was a major factor in portal hypertension. Semiautomatic determination of the fractional area of sinusoidal collapse on chromotrope-stained sections and automatic measurement of Sirius red-stained collagen fiber density were performed. Hepatic venous pressure gradient significantly correlated with fractional sinusoidal collapse area (r = 0.61, p less than 0.001) and with Sirius red-stained collagen fiber density (r = 0.43, p less than 0.01). We conclude that portal hypertension in the course of acute hepatitis is related to the severity of liver damage and is a major factor in the development of ascites. Portal hypertension is mainly determined by intrahepatic vascular space being reduced by the collapse of sinusoids.