Thin films of symmetric diblock copolymers confined between two hard, flat and homogeneous surfaces have been investigated by means of Monte Carlo simulations on a simple cubic lattice. For such simulations, the match between bulk lamellar period L-0 and the simulation box size is crucial to obtain meaningful results. The simulations have been performed in an expanded grand-canonical ensemble, where the chemical potential and the temperature of the confined films are specified and the density is allowed to fluctuate. The dependence of morphology, density, and chain conformation in the confined films on the type of surfaces, surface separation, and the strength of surface-block interactions has been studied systematically. Our results are consistent with experimental findings.