BACKGROUND Mesenteric venous thrombosis is a rare cause of acute abdominal pain that may be the result of coagulation abnormalities. METHODS Four consecutive patients with mesenteric venous thrombosis underwent haematological evaluation. RESULTS All four had activated protein C resistance resulting from a single mutation in the gene coding for coagulation factor V. Three had surgery; in one patient the diagnosis was made by ultrasonography. One of the patients who had surgery died but the other three survived and were treated with long-term anticoagulation. CONCLUSIONS Activated protein C resistance may be an important pathogenetic factor in primary mesen-teric vein thrombosis.