We develop one-loop effective vertices and propagators in the linear sigma model at finite temperature satisfying the chiral Ward identities and use these in turn to compute the pion dispersion relation in a pion medium for small momentum and temperatures on the order of the pion mass at next to leading order in the parameterm&pgr;2/4&pgr;2f&pgr;2and to zeroth order in the parameter(m&pgr;/m&sgr;)2.We show that this expansion reproduces the result obtained from chiral perturbation theory at leading order. The main effect is a perturbative, temperature-dependent increase of the pion mass. ©2001 American Institute of Physics.