Blood coagulation is initiated when tissue factor binds to coagulation factor VIIa to give an enzymatically active complex which then activates factors IX and X, leading too thrombin generation and clot formation. We have determined the crystal structure at 2.0-Angstrom resolution of active-site-inhibited factor VIIa complexed with the cleaved extracellular domain of tissue factor. In the complex, factor VIIa adopts an extended conformation. This structure provides a basis for understanding many molecular aspects of the initiation of coagulation.