The resolution of a muscular hematoma is an important step in the recovery from a muscle injury. Because heat application has been advocated to accelerate this resolution, the effectiveness of microwave diathermy as a means of selectively heating the musculature was studied. For this purpose muscular hematomas were formed in 6 pigs by bilateral injections of blood labeled with the radioisotope chromium-51 (Cr51) into the biceps femoris muscle. One hematoma site was heated with microwave diathermy and the opposite side served as a control. The tissue temperature at the hematoma site was shown to be in the therapeutic range of 42 to 45C, which would cause a maximal local blood flow response. A decay curve was formed by counting the radioactivity of the hematoma site with a scintillation counter. Best fit lines fitted to the decay curves to determine effective half-life values showed that the treated side had a significantly shorter washout time than the control side. This result supports the use of heat as an adjunct to other therapy aimed at resolution of muscular hematomas.