The emissivity of spontaneous synchrotron radiation is computed for a plasma consisting of a background thermal plasma in addition to an energetic runaway electron component. The analysis is performed for both the ordinary and extraordinary modes, for frequencies in the vicinity of the electron plasma frequency and the higher harmonics of the electron gyrofrequency, and for &ohgr;e≲&OHgr;e(where &ohgr;eand &OHgr;eare the electron plasma and cyclotron frequencies, respectively). The relativistic gyroresonance with the runaway electrons is found to result in a level of spontaneous emission which, for frequencies in the neighborhood of &ohgr;e, is significantly enhanced over the thermal radiation.