The oral vitamin intakes and concentrations of vitamins in blood of eight children on long-term peritoneal dialysis and six control children were measured. All patients received a daily supplement containing water-soluble vitamins. Serum concentrations of vitamin A, vitamin B-12, ascorbic acid, and folic acid and dialysate concentrations of ascorbic acid were determined. Thiamin and riboflavin were assessed by measuring erythrocyte enzyme activities. Vitamin B-6 was measured as plasma pyridoxal phosphate. Dietary vitamin intake was determined with weighed 3-d food records. The dialysis patients had significantly greater stores of vitamin A, thiamin, riboflavin, pyridoxal phosphate, and folic acid than did the control population (P less-than-or-equal-to 0.01). The patients' combined dietary and supplemental intake of all vitamins except ascorbic acid was also significantly greater than the intake of the control group (P < 0.01). Vitamin supplementation is associated with normal or greater-than-normal values of water-soluble vitamins in pediatric patients receiving long-term peritoneal dialysis.