The analysis of the mechanical properties of thin films by nanoindentation has been recently subject of numerous theoretical and phenomenological studies as well as numerical simulations. In this work, we report on the application of the Winkler elastic foundation theory to analyze atomic force microscope nanoindentations of poly(n-butyl methacrylate) films with thicknesses of 90, 196, and 485 nm. The elastic moduli of the samples were found to be 1.13 +/- 0.43 GPa, 1.34 +/- 0.32 GPa and 1.23 +/- 0.24 GPa, respectively, after indentations of at least 50% of the film thickness. These data rely on the independent determination of the mechanical properties of 485-nm thick films, using Sneddon's model at low penetration depth (yielding 1.27 +/- 0.37 GPa). Our data show that the substrate effects begins to be noticeable only after indenting to a depth of more than 40% of the film thickness. POLYM. ENG. SCI., 51: 1507-1512, 2011. (C) 2011 Society of Plastics Engineers