We investigate experimentally the lifetime of the photons in a cavity containing a medium exhibiting strong positive dispersion. This intracavity positive dispersion is provided by a metastable helium gas at room temperature in the electromagnetically induced transparency regime, in which light propagates at a group velocity of the order of 104 m.s -1.The results definitely prove that the lifetime of the cavity photons is governed by the group velocity of light in the cavity and not its phase velocity. © 2011 Optical Society of America.