189 patients with various types of glomerular disease were studied. Creatinine clearance, protein excretion and urinary excretion of fibrin degradation products (FDPs) were measured before and at various intervals (up to 42 months) after starting treatment with indomethacin, alone or in combination with other drugs. The following observations were made: a. Patients with a urinary FDP in excess of 2 mg/24 hours before treatment had a significantly lower creatinine clearance and a significantly higher protein excretion than patients excreting less than 2 mg FDP/24 hours, indicating that FDP excretion reflects the severity of the renal disease. b. During treatment, the incidence of high FDP excretion decreases progressively, but remains high in patients who ultimately develop renal insufficiency. c. There is no correlation between the initial value of FDP excretion and the subsequent changes in creatinine clearance and proteinuria during treatment. This implies either that the initial FDP excretion has no prognostic value or, perhaps more likely, that disease activity is modified by treatment. d. The best correlation between FDP excretion and evolution is found in proliferative glomerulonephritis. There are reason to suppose that, in this group at least, the treatment influenced the evolution of the disease.