Background and Objectives: In Crohn's disease (CD), belonging to inflammatory bowel disease, the small intestine is involved in most cases. Most frequently affected is the distal ileum, where vitamin B-12 is specifically absorbed. Therefore, malabsorption of vitamin B-12 is quite likely to occur in patients with CD. In this study, we have studied the vitamin B-12 status in CD patients. Methods and Study Design: Forty eight patients with CD were evaluated for their food intake, and circulating concentrations of vitamin B-12, folic acid, and homocysteine (Hcy) as a sensitive marker for the insufficiency of these vitamins and a risk factor of atherosclerosis. Results: Plasma Hcy concentration was significantly correlated with serum vitamin B-12 concentration alone, and 60.4 % of the subjects had hyperhomocysteinemia. Receiver Operating Characteristics (ROC) analysis showed that serum concentration of vitamin B-12, but not folic acid, predicted hyperhomocysteinemia. Their intake of vitamin B-12 was much higher than the Japanese RDA, but not correlated with blood concentrations of vitamin B-12 or Hcy, probably due to malabsorption. Conclusions: Vitamin B-12 insufficiency and hyperhomocysteinemia were highly prevalent in CD patients. Recently, the significance of extra-intestinal complications of CD has been increasingly recognized, and our finding is likely to be of clinical importance.