  The thermodynamics of massless phi^4-theory is studied within screened
perturbation theory (SPT). In this method the perturbative expansion is
reorganized by adding and subtracting a mass term in the Lagrangian. We
analytically calculate the pressure and entropy to three-loop order and the
screening mass to two-loop order, expanding in powers of m/T. The truncated
m/T-expansion results are compared with numerical SPT results for the pressure,
entropy and screening mass which are accurate to all orders in m/T. It is shown
that the m/T-expansion converges quickly and provides an accurate description
of the thermodynamic functions for large values of the coupling constant.
