To clarify the prevalence of protein C deficiency in patients with cardiovascular problems, we identified 54 patients with protein C deficiency by screening approximately 26,800 patients. The observed prevalence of protein C deficiency was 1 in 500. From the results of genetic analysis of 67 Japanese families with protein C deficiency, the recurrent defects of Phe139Val, Arg169Trp, Val297Met, Met364Ile substitutions and a G8857 deletion were accounting 49% of Japanese families with protein C deficiency. The defects including Phe139Val and Met364Ile substitutions and a G8857 deletion were only found in Japanese subjects and seemed to be a founder effect. Protein C deficiency is associated with a predisposition to venous thrombosis. It is not clear whether the deficiency is involved in arterial occlusion. We identified 34 patients with protein C deficiency manifested 45 episodes of arterial occlusive disease. We compared the onset age of arterial occlusive disease between patients with protein C deficiency and a group of patients with normal protein C levels. We found the early onset of acute myocardial infarction and atherothrombotic cerebral infarction in patients with protein C deficiency. Thus, our study suggests that congenital protein C deficiency contributes to earlier onset of arterial occlusive diseases in Japanese subjects.