This study assesses the usefulness of ultrasound in the diagnosis of portal hypertension due to hepatic cirrhosis. Seventy-nine patients with portal hypertension and 45 control subjects underwent ultrasonography. Two factors were measured: (a) the caliber of the portal vein and (b) the caliber variation of the splenic and superior mesenteric veins during respiration. A lack of normal caliber variation (an increase during inspiration and a decrease during expiration) in these vessels is put forward as an ultrasonographic sign of portal hypertension, and the pathophysiological and clinical significance of this finding are discussed. The sensitivity of ultrasound in detecting portal hypertension, based on the measurement of caliber variation, was 79.7%, and the specificity was 100%. In contrast, the sensitivity of the method, assessed on the basis of portal dilatation, was only 41.8%.