In severe multipath environments the relevant antenna parameter for mobile radiofrequency system performance is nor more its radiation pattern but rather its radiation efficiency. Unfortunately this feature is seldom known from the antenna manufacturer and is also difficult to calculate accurately for practical antennas. In this paper, the determination of the radiation efficiency of an antenna from measurements carried out in the stochastic fields of a mode stirred reverberation chamber is described and discussed. Using a thin monopole antenna on circular ground plane as a reference antenna, the efficiency of a horn antenna is measured at 1800MHz. The influence on the measured radiation efficiency of some direct coupling between the antenna under test (AUT) and the source antenna is discussed