The phonon spectrum of sodium is calculated in the harmonic approximation. The effective interaction between the ions is separated into direct two-body forces between bare ions and effective attraction due to the presence of conduction electrons. Careful treatment of the two-phonon processes includes the calculation of the electron-phonon scattering based on the pseudo-potential method and the screening effects due to the interaction between electrons by the Hartree-Fock approximation with a screened exchange potential. Comparison of the results for sodium with neutron diffraction measurements and a previous calculation by Toya shows good agreement on the whole. It is hoped that this method may be applied to some other metals.