Previous studies of tumor cell-associated procoagulants and fibrinolytic factors have strongly suggested that local thrombin and plasmin generation may be important in tumor growth and dissemination. Given that one central target of both of these serine proteases is fibrin(ogen), a logical extension of this hypothesis is that local fibrin deposition and dissolution may be key determinants of tumor progression. In this paper, the role of fibrin(ogen) and its degradation products in the growth and spontaneous metastasis of Lewis lung carcinoma was directly examined by comparative studies of control and fibrinogen-deficient mice. Fibrinogen deficiency was found to have no effect on the time required for the formation of palpable tumors, tumor angiogenesis, overall tumor architecture, or primary (s.c.) or secondary (pulmonary) tumor growth. However, fibrinogen deficiency markedly reduced the incidence of spontaneous macroscopic metastases in the lung and regional lymph nodes, a process that occurred relatively late in tumor development. Furthermore, a significant quantitative reduction in pulmonary micrometastases was observed in fibrinogen-deficient mice. Quantitative analyses of pulmonary micrometastases in primary tumor-bearing mice indicated that spontaneous showering of tumor cell emboli into the lung was robust, regardless of animal genotype. Hence, our results suggest fibrin(ogen) plays an important role in spontaneous metastasis, facilitating the stable adhesion and/or survival of metastatic emboli after tumor cell intravasation. These studies suggest that therapeutic strategies focusing on hemostatic factors may be effective in controlling solid tumor metastasis, particularly if used for the treatment of micrometastatic disease.