This study was designed to check whether lowering plasma homocysteine concentration resulting from treatment is associated with decreased thrombotic activity in haemodialyzed patients. The study group included 38 patients undergoing chronic haemodialysis. Blood was collected after an overnight fast after two days of weekend intervals in haemodialysis. Tests were performed before and after eight weeks of combined treatment with vitamin B6 (100 mg/day, oral), vitamin B12 (1 mg s.c. once a week) and folic acid (15 mg/day, oral). Homocysteine, vitamin B12 and folic acid concentrations in serum, antithrombin, fibrinogen and protein C activity in plasma and thromboxane B2 levels in activated platelets were determined before and after treatment. Combined treatment resulted in a statistically significant decrease in plasma homocysteine concentration. No influence of this finding on parameters of thrombotic activity was found. The following conclusions were drawn: (1) Treatment with physiological doses of vitamin B12, B6 and folic acid effectively decreases homocysteine concentration in haemodialyzed patients with chronic renal failure. (2) Monitoring of treatment with concentrations of folic acid and vitamin B12 is unfounded as no significant correlations with homocysteine were revealed. (3) The decrease in homocysteine concentration in haemodialyzed patients does not affect thrombotic activity. Therefore, treatment with physiological vitamin doses should not be recommended for routine use.