SUMMARY Observations on 362 determinations of urinary fibrin degradation products (FDP) and 163 serum FDPs in 39 patients are presented. Normal kidney donors had no significant urinary or serum FDP before or after surgical trauma (mean, 0.01 μg/ml) before and after surgery. Patients with advanced glomerulonephritis had significantly higher urinary FDP levels (mean, 80.83 μg/ml) than those with chronic pyelonephritis (mean, 14.62 μg/ml) (P<0.025). In renal allotransplanted patients, urinary FDP levels were significantly higher during rejection episodes (mean, 33.14 μg/ml) when compared to those without any rejection (mean, 0.02 μg/ml) (P<0.001). Serum FDP was not significantly elevated during allograft rejection. Serial urinary FDP measurements were of value in detecting 9 of 10 acute rejection episodes studied. Partial ureteral obstruction and acute tubular necrosis were not accompanied by elevated urinary FDPs except in the presence of proven concurrent rejection episodes.