Phonon-wall collisions (with smooth or rough walls) have a deep influence on the thermal conductivity of nanowires. Usually this influence is analyzed in the steady-state thermal conductivity. Here, by using a phonon-hydrodynamic model with slip heat flow along the walls, we explore the influence of phonon-wall collisions on frequency-dependent thermal conductivity, which imply a reduction of it with increasing frequency. Understanding of this dependence would allow one to obtain information on the collisions complementary to that of the steady states. © 2011 American Institute of Physics.