We have investigated the structural, dynamical, and dielectric properties of A-La2O3, within density-functional theory. The relaxed structural parameters are found to be in very good agreement with experimental data. Density-functional perturbation theory was used to calculate the zone-center phonon frequencies, Born effective charge tensors, and the dielectric permittivity tensors. The static permittivity constant is decomposed to its electronic component and the individual contribution of IR-active modes. It is found, for both the direction parallel to the trigonal axis and the direction perpendicular to it, that the ionic contribution to the static dielectric permittivity is mostly due to a single IR-active mode.