  We evaluate the shear viscosity of a pion gas in the relativistic kinetic
theory approach. The in-medium propagator of the $\rho$ meson at finite
temperature is used to evaluate the $\pi-\pi$ scattering amplitude in the
medium. The real and imaginary parts of the self-energy calculated from
one-loop diagrams are seen to have noticeable effects on the scattering
cross-section. The consequences on temperature dependence of the shear
viscosity evaluated in the Chapman-Enskog and relaxation time approximations
are studied.
