Serum levels of oxalate are elevated in uremic patients on dialysis. The effect of living related donor kidney transplants on serum and urine oxalate levels was studied in 8 patients. Serum and urine oxalate levels were measured prior to transplant, on the day of transplant and daily for 5 days postoperatively, and the results compared to those in 11 normal subjects. All transplanted kidneys functioned immediately. Serum oxalate fell from 55 +/- 9 mumol/l (484 +/- 79 micrograms/dl) before transplant to 21 +/- 3 mumol/l (185 +/- 26 micrograms/dl) the day after transplant, and to 9 +/- 2 mumol/l (79 +/- 18 micrograms/dl) 72 h after transplant. Serum oxalate in normal subjects was 9 +/- 2 mumol/l (79 +/- 18 micrograms/dl). During the initial 24 h after transplant urine oxalate averaged 1,244 +/- 150 mumol/l (109.5 +/- 13.2 mg), but fell to levels not statistically different from normal by 72 h after transplant. Rapid clearance of oxalate after transplant leads to transient hyperoxaluria until normal levels of serum oxalate are reached.