Measurements of the magnetostrictive stress in epitaxial Fe films on W(1 0 0) reveal a sharp deviation of the magnetoelastic coupling in nmn films from bulk magnetostriction. Fe films thinner than 20 nm contract upon magnetization in [1 0 0], whereas thicker films expand upon magnetization in [1 0 0]. In situ stress measurements are analyzed to correlate the film thickness dependent magnetoelastic coupling with the thickness dependence of film strain. A strain dependent contribution to the magnetoelastic coupling is found to describe the experimental data for films thicker than 15 nm.