Abstract <p>The distributed receptivity of a swept-wing laminar boundary layer to unsteady free-stream vortices with a streamwise-oriented vorticity vector in the presence of spanwise-uniform surface roughness has been investigated experimentally. The experiments were performed on a 25-degree swept-wing model under fully controlled disturbance conditions. It has been found that unsteady streamwise flow vortices can lead to very efficient streamwise-distributed excitation of unsteady cross-flow instability modes at combination spanwise wavenumbers due to vortex scattering on surface roughness. This paper (Part 1 of the study) describes the experimental approach and its theoretical justification, the base-flow structure, experimental evidence for the high efficiency of the receptivity mechanism, and the experimental confirmation and importance of streamwise wavenumber resonance. Part 2 of this study is devoted to the experimental estimation of the amplitudes and phases of distributed vortex-roughness receptivity coefficients as functions of the disturbance frequency and spanwise wavenumber. It also presents the receptivity coefficients responsible for the excitation of cross-flow waves on a&#xa0;smooth surface and provides a comparison of the relative efficiency of these two mechanisms.</p>