The elastic behaviour of grains bordering a free surface is analysed through the self-consistent grain-matrix interaction model. The approach proposed here assumes in-plane stresses and free Poisson expansion or contraction in the normal direction; in-plane grain-matrix interaction remains active for surface grains. Grains at greater depths interact with the matrix in the usual way, thus producing two distinct results for the surface and for the bulk. The effective diffraction response for surface limited X-rays is the attenuation-length weighted average of surface grains and bulk grains. The existing self-consistent models can be easily adapted to the surface effect to include preferred orientation and non-spherical grains. An important consequence of the surface effect is that materials consisting of elastically anisotropic grains exhibit nonlinear lattice strain versus sin2 ψ distributions even without texture. In practice, the effect can be minimized by large tilt angle ranges, penetration depths larger than the grain size, or under-sampling lattice strains at low tilt angles.