In patients with portal hypertension the azygos system collects the main part of superior portosystemic shunts; accordingly, azygos blood flow might be a reflection of this collateral circulation. Azygos blood flow, estimated by the continuous thermodilution method with a catheter inserted into the azygos arch, was measured in 20 patients with cirrhosis. The variability of baseline measurements was 6.9 +/- 3.0 p. 100 and the reproducibility was 5.7 +/- 5.8 p. 100. In patients with cirrhosis, azygos blood flow ranged from 0.22 to 1.25 l/min (0.64 /+- 0.30 l/min; mean +/- SD) and was significantly higher than in patients without portal hypertension (0.13 +/- 0.04 l/min). In this series of patients, azygos blood flow was significantly correlated with the hepatic venous pressure gradient but neither with hepatic blood flow nor with cardiac output. This study shows that azygos blood flow may be estimated with the continuous thermodilution method and that azygos blood flow is approximately six times higher in cirrhotic patients than in controls. This result suggests that blood flow through the superior portosystemic shunts is very high.