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 μmol/l (484 ± 79 μg/dl) before transplant to 21 ± 3 μmol/l (185 ± 26 μg/dl) the day after transplant, and to 9 ± 2 μmol/l (79 ± 18 μg/dl) 72 h after transplant. Serum oxalate in normal subjects was 9 ± 2 μmol/l (79 ± 18 μg/dl). During the initial 24 h after transplant urine oxalate averaged 1,244 ± 150 μmol/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 o