Vitamin B6 is very important for the normal function of multiple organ systems. In the majority of patients with chronic renal failure and in patients during various forms of renal replacement therapy can develop vitamin B6 deficiency from many causes. Intravenous administration of 20 mg furosemide led to the increase of urinary excretion and fraction excretion (FE) of vitamin B6 in patients with chronic renal failure. This is a new side effect of furosemide. The daily oral dose of pyridoxine 6 mg was optimal for the patients without erythropoietin (EPO) treatment during the period of 12 months of CAPD. Erythrocyte vitamin B6 was determined by an indirect method, that is, by measuring the effect of pyridoxal-5-phosphate (PLP). In the other group of CAPD patients plasma vitamin B6 was in the reference range, and the mean value of peritoneal clearance of vitamin B6 was very low: 8.8% of urea clearance. In addition, an indirect relationship between the effect of PLP and plasma vitamin B6 was found. Indirect evidence has shown that erythrocyte vitamin B6 is consumed by the hemoglobin synthesis much more during EPO treatment in hemodialysis patients. No influence of pyridoxine 5 to 6 mg/day on decreased parameters of cellular immunity was found. For prevention of vitamin B6 deficiency in hemodialysis and CAPD patients we recommend the following doses of pyridoxine: for patients without EPO treatment 5 mg/day, and with EPO treatment 20 mg/day. A favorable effect of pyridoxine 50 mg/day has also been found on several parameters of cellular immunity in hemodialysis patients.