Depression may be associated with elevated homocysteine (HCY) levels. Procedures aiming at its decrease, i.e. supplementation with folic acid or vitamin B12, have antidepressant effect. Both depression and elevated HCY can increase cardiovascular risk. In this study, clinical and biochemical factors, including markers of endothelial function, in relation to hyperhomocysteinemia, in patients with bipolar depression during acute episode were studied. Method: One hundred and twelve patients (24 male, 88 female), aged 20-78 (mean 51 ± 14 years), with depressive episode in the course of bipolar mood disorder have been included. The assays were made of serum concentrations of HCY, vitamin B12, folic acid as well as markers of endothelial function such as E-selectin and intracellular adhesion molecule-1 (ICAM-1). Results: Hyperhomocysteinemia (>15 mM) was found in 50 patients (45%), significantly more frequently in male (67%) than in female subjects (39%). Female patients with hyperhomocysteinemia were significantly older than the remaining ones. A significant inverse correlation between HCY level and concentration of folic acid and vitamin B12 as well as with E-selectin and ICAM-1 was observed. Conclusion: The results point to a significant prevalence of hyperhomocysteinemia in bipolar depressed patients during an acute episode. They also corroborate the correlation between increased concentration of HCY and lower level of vitamin B12 and folic acid. An unexpected finding of negative correlation of HCY level with markers of endothelial functions in such patients is discussed in view of current concepts of the role of HCY in various conditions.