[en] A theory of free-carrier absorption is given for quantum wires when carriers are scattered by boundary roughness and the radiation field is polarized along the length of the wire. The free-carrier absorption coefficient is found to be an oscillatory function of the photon frequency and of the width of the wire. The obtained results are compared with different scattering mechanisms for quasi-one-dimensional structures. It is found that boundary roughness scattering is important, especially when the width of the wire and the temperature decrease. In addition, it was found that in quantum wires the electron-boundary roughness interaction gives a greater contribution to the absorption than the electron-acoustic phonon interaction. The results are interpreted in terms of boundary roughness-assisted transitions between size quantized subbands