Fibrin was detected by immunospecific techniques associated with both intravascular and extravascular tumor deposits in renal cell carcinoma. In addition, coagulation factors VII and X were demonstrated in intercellular spaces within tumor tissue and adjacent to the surface of tumor cells. Scant accumulation of platelets on intravascular tumor masses was observed. These data suggest that tumor cells in renal cell carcinoma may induce fibrin formation locally by a factor VII-mediated (and thus tissue factor-initiated) pathway of blood coagulation. This mechanism may also account for the hypercoagulable state that exists with this tumor type. We postulate that local fibrin formation may contribute to the growth and spread of this particular type of cancer and that the course of renal cell carcinoma may be ameliorated by anticoagulant therapy.