Using off-axis magnetron sputtering onto metallic Nb-SrTiO3 substrates, we have grown a series of epitaxial c-axis oriented PbTiO3 perovskite films with thicknesses ranging from 460 A ̊ down to 12 A ̊ (about 3 unit cells). Topographic measurements using atomic force microscopy showed that these films are essentially atomically smooth. X-ray measurements allowed us to precisely determine the thickness of the films and the c-axis lattice parameter value, and to confirm epitaxial growth. It is found that the c-axis lattice parameter systematically decreases with decreasing film thickness.