The clotting values of 50 patients with glomerulonephritis were examined. Three different coagulation groups were recognised: those with normal clotting values (group 1); those with high concentrations of factor VIII but otherwise normal clotting results (group 2); and patients who showed the presence of an activator of the intrinsic coagulation pathway, indicated by the presence of a short activated partial thromboplastin time or the ability of patients' plasma to shorten control clotting time in mixing studies (group 3). Patients in group 2 either had a uniform rise in all three components of the factor VIII molecule or a disproportionately higher concentration of factor-VIII-related antigen. In contrast, the level of VIII clotting activity in patients in group 3 was always higher than concentrations of either VIIIAg or VIIIWF. A significantly high incidence of thrombotic complications was observed in patients with group 3 but in none of the patients in either group 1 or group 2. Impaired renal function was more common in patients in groups 2 and 3, with higher mean serum creatinine concentrations in those with group 3. Patients with glomerulonephritis who have a short partial thromboplastin time with kaolin or who shorten control clotting time form a subgroup in whom hypercoagulation could adversely affect the course of their disease. The value of antiplatelet or anticoagulant treatment in these patients needs to be explored.