Electromagnetically induced transparency (EIT) in a three-level -system is a phenomenon leading to high dispersion and thus significant changes in the group velocity inside a medium. Some authors have proposed introduction of such highly dispersive medium into a cavity in order to reduce its linewidth, or enhance the sensitivity of an interferometer. These proposals are usually made considering that the phase velocity rules the decay rate of the cavity [1]. But other studies assume that this decay time is connected to the group velocity [2]. In the light of the above discrepancy, we investigate experimentally the decay time of a cavity in the presence of positive or negative dispersion. © 2011 IEEE.