Rapidly progressive glomerulonephritis (RPGN) was diagnosed in six patients on the basis of the clinical course and typical pathological features on renal biopsy. These patients were given heparin sodium intravenously; when their renal function had stabilized, four patients were then given anticoagulant and antithrombotic agents orally. The administration of heparin was associated with increases in urine output and creatinine clearance in five patients; the orally administered agents resulted in additional improvement of renal function in three of four patients. All patients suffered hemorrhagic complications; one died and another became functionally anephric. Immunofluorescent studies revealed three different deposition patterns. The presence of fibrinogen deposition in glomeruli did not correlate with response to anticoagulant therapy. Anticoagulant and antithrombotic therapy appears to arrest and partially reverse the course of RPGN in most patients, although hemorrhagic complications are frequent.