Epitaxial (001) Co₉₀Fe₁₀/Cu multilayers were grown by sputter deposition on MgO(001) substrates using bilayer seed layers containing thin Fe and Pt layers. The influence of the thickness of the Fe layer on the structure of the Co₉₀Fe₁₀/Cu multilayers was studied by x-ray diffraction, and compared to its effect on the giant magnetoresistance measured in these films. The structural quality, especially of the Co₉₀Fe₁₀/Cu layers closest to the seed bilayer, and the change in magnetoresistance depend sensitively on the Fe seed layer thickness. The largest giant magnetoresistance effect is found for Fe thicknesses around 12 Å. The influence of the Fe layer thickness is discussed in terms of lattice matching between the substrate, Fe, Pt, and Co₉₀Fe₁₀/Cu layers, and alloying of Fe and Pt in the seed layer.