A close temporal relationship between higher levels of portal pressure during the night and the peak incidence of acute variceal bleeding has recently been demonstrated in patients with cirrhosis. Because hemodynamic changes may have a role in triggering this hemorrhagic episode, we measured systemic and portal hemodynamic parameters at 4-hr intervals for 24 hr in 12 cirrhotic patients. These results were compared with those obtained in eight healthy subjects. Cardiac output, femoral and portal blood flows were measured by Doppler technique. In cirrhotic patients, heart rate and mean arterial pressure remained constant throughout the whole study period. A marked and significant increase in portal blood flow (917 ± 248 ml/min) (mean ± S.D.) as compared with mesor values (649 ± 114 ml/min, p < 0.001) was observed at midnight. This effect was accompanied by a mild but significant rise of cardiac output (from 6.5 ± 0.7 to 6.8 ± 0.7 l/min, p < 0.01) at 2400 hr. A significant correlation between both hemodynamic parameters was found (r = 0.78, p < 0.01). Cosinor analysis showed a significant (p < 0.05) circadian rhythm for both portal blood flow and cardiac output with an acrophase at 0050 hr. In the healthy subjects group, a significant decrease of mean arterial pressure and heart rate was observed at 2400 hr. Cosinor analysis confirmed the presence of significant rhythm for both hemodynamic parameters. In contrast with cirrhotic patients, no significant changes were observed in portal blood flow and cardiac output in healthy subjects. Our results show that in patients with cirrhosis, maximal increases in portal blood flow occur at night. The circadian variation of portal blood flow could have important implications in the management of these patients. (Hepatology 1994;20:1198–1203).