Abstract: The prescription of multivitamin supplements for dialysis patients is routine practice, but the doses prescribed differ greatly from one dialysis center to another. Few data are available concerning long-term vitamin supplementation and its effects on patients either on high-flux hemodialysis or receiving postdialysis supplementation. For several years, we have systematically prescribed to our patients an oral postdialysis multivitamin supplement containing thiamine hydrochloride 100 mg, riboflavin 20 mg, pyridoxine hydrochloride 50 mg, folic acid 6 mg, and ascorbic acid 500 mg. The aim of this study was to perform a cross-sectional long-term evaluation of the vitamin levels in patients who received this vitamin supplement for at least 12 months. We also were interested in investigating the plasma oxalic acid and total homocysteine levels associated with the long-term prescription of these vitamin supplements. Thirty-three patients on high-flux dialysis were studied. Vitamin levels and/or vitamin-dependent enzymatic activities were within the normal range (N) in all patients. The mean results (±SD) were plasma ascorbic acid 13.6 ± 6.4 mg/L (N > 4), plasma folate 14.1 ± 1.1 μg/L (N > 3), for vitamin B1, α-ETK 1.02 ± 0.02 (N < 1.18) and ETKo 100 ± 13 U/L (N > 70), for vitamin B2, α-EGR 1.00 ± 0.07 (N < 1.52) and EGRo 1282 ± 213 U/L (N > 672), and for vitamin B6, α-EGOT 1.34 ± 0.10 (N < 1.8) and EGOTo 380 ± 84 U/L (N > 228). Plasma oxalic acid was higher than normal in all patients (mean = 61 ± 15 μmol/L, N < 33). However, all patients had oxalic acid levels within the range reported in the literature for patients not taking extra ascorbic acid. Mean total homocysteine was 24 ± 8 μmol/L with only 4 patients (12%) having normal levels (N < 15). In conclusion, the postdialysis supplement given provides adequate vitamin levels in almost all patients in the long term. Postdialysis prescription allows an optimal compliance with the treatment, is well accepted by the patients, and is cost-effective.