The authors report the case of a 51 year-old man, without any personal or familial history of thromboembolism, presenting with abdominal pain. Portal vein thrombosis was demonstrated by ultrasonography and arteriography. The patient had neither esophageal varices or congestive gastropathy. No cause for portal vein thrombosis was detected. Type I protein C deficiency was demonstrated in this patient as well as in his asymptomatic sister. The presence of a (fortuitously?) associated increase in platelet aggregability initially led to a trial regimen of aspirin (300 mg per day); abdominal pain resolved, and a partial regression of portal vein thrombosis was demonstrated on ultrasonograms six months later; no further complications occurred during the 4-year follow-up period. The 13 previously published cases of protein C deficiency-associated portal vein thrombosis are reviewed.