  Hadron resonance gas models provide a good description of the equation of
state of quantum chromodynamics determined by lattice QCD calculations at
temperatures $T \sim 100-155$ MeV. In this paper we investigate the effects of
an exponentially increasing hadron mass spectrum (Hagedorn spectrum) on the
azimuthal anisotropy of the rapidly expanding matter formed in
ultrarelativistic heavy ion collisions. If the temperature at which the
conversion from fluid degrees of freedom to hadrons is sufficiently close to
the Hagedorn temperature, the production of Hagedorn resonances suppresses the
differential elliptic flow of all hadron species.
