Inappropriate blood coagulation plays a central role in the onset of myocardial infarction, stroke, pulmonary embolism, and other thrombotic disorders. The ability to screen for an increased propensity to clot could prevent the onset of such events by appropriately identifying those at risk and enabling prophylactic treatment. Similarly, the ability to characterize the mechanical properties of clots in vivo might improve patient outcomes by better informing treatment strategies. We have developed a technique called sonorheometry. Unlike existing methods, sonorheometry is able to assess mechanical properties of coagulation with minimal disturbance to the delicate structure of a forming thrombus. Sonorheometry uses acoustic radiation force to produce small, localized displacements within the sample. Time delay estimation is performed on returned ultrasound echoes to determine sample deformation. Mechanical modeling and parametric fitting to experimental data yield maps of mechanical properties. Sonorheometry is well suited to both in vitro and in vivo applications. A control experiment was performed to verify that sonorheometry provides mechanical characterization in agreement with that from a conventional rheometer. We also examined thrombosis in blood samples taken from four subjects. This data suggests that sonorheometry may offer a novel and valuable method for assessing the thrombogenicity of blood samples.