We propose a new perturbation approach to finite-size effects within t
he rho(4) field theory for a one-component order parameter with period
ic boundary conditions. Our approach is applicable both above and belo
w T-c. Renormalization-group calculations of finite-size scaling funct
ions in three dimensions are compared with new Monte Carlo data for th
e L x L x L Ising model with L = 8, 16, 32. The field-theoretic predic
tions are in good overall agreement with the Monte Carlo data.