The aim of this paper was to assess diurnal periodicity in the time pattern of the time of onset of acute gastrointestinal bleeding related to portal hypertension manifested by hematemesis or melena in cirrhotic patients over a period of 24 h. The study was a prospective evaluation of separate and consecutive episodes of bleeding requiring admission to hospital following hematemesis or melena in cirrhotic patients with the use of cosinor statistical analysis to determine rhythmicity. A total 744 episodes of bleeding were studied during separate and consecutive hospital admissions over a 68-month period. The time of onset of manifestation of variceal bleeding (n = 608) in both alcoholic (n = 279) and non-alcoholic (n = 329) cirrhosis and bleeding from non-variceal sources excluding peptic ulcer (n = 136) showed a significant diurnal rhythm (P = 0.005 and P < 0.05, respectively), with two peaks at 08:00 h and 20:00 h. This pattern was not modified by sex, age, severity of liver disease, seasonal variations or initial presentation with hematemesis or melena on its own. This is the first study showing significant diurnal periodicity in the time of onset of bleeding in cirrhotic patients. It is also the first study to show periodicity of upper gastrointestinal bleeding from any source. The cause for the observed rhythmicity is not apparent but parallel changes in portal pressure and/or changes in hemostatic factors could explain this observation.(ABSTRACT TRUNCATED AT 250 WORDS)