Background and Aims: HOMOCYSTEINE is a sulphur amino acid derived from methionine. Epidemiological studies show an association between hyperHOMOCYSTEINEmia and an increased cardiovascular risk, a fact that has also been confirmed in patients with chronic renal failure. This study, conducted in stage 5 chronic kidney disease patients, seeks to define the prevalence of hyperHOMOCYSTEINEmia, evaluate the clearance of HOMOCYSTEINE with dialysis, and describe the frequency of the mutation in the mutilen-tetrahydrofolate reductase enzyme and its relationship with plasma levels of HOMOCYSTEINE.Methods: The reduced percentage of HOMOCYSTEINE and clearance of urea were analysed every six months for seven years in patients on dialysis. Urea and total HOMOCYSTEINE in plasma were measured in each of these studies and the type of dialyser - low or high permeability - used and the dialysis duration was determined.A molecular study of the gene coding for mutilen-tetrahydrofolate reductase enzyme was carried out in a group of patients and any C-T point mutation at position 677 of this gene was investigated. Mutation was described as not present, heterozygous for this mutation or homozygous for lactation.Results: Neither the average levels of HOMOCYSTEINE before and after dialysis or the reduction percentage of HOMOCYSTEINE varied with gender, although purification of urea was higher in women. Comparisons of HOMOCYSTEINE levels and percentage reduction in this ratio according to the ultra filtration of the dialyser used showed significant results. The molecular study of the gene in mutilen-tetrahydrofolate reductase enzyme showed that mutation was present in 54.8%: 45.2% with heterozygous polymorphism and 9.7% with homozygous.Conclusions: Patients undergoing hemodialysis were found to have higher levels of urea and its clearance was greater with the higher the ratio of ultra filtration dialyser. Mutation of the gene in mutilen-tetrahydrofolate reductase enzyme was similar in our patients compared to the general population and had no impact on plasma levels of HOMOCYSTEINE.