We study the band-gap renormalization m a model semiconductor quantum wire due to the exchange-correlation effects among the charge carriers. We construct a two-subband model for the quantum wire, and employ the GW-approximation to obtain the renormalized quasi-particle energies at the optical band edge. The renormalization is calculated as a function of electron-hole plasma density and the wire radius. Our results show that the very presence of the second subband affects the renormalization process even in the absence of occupation by the carriers. We compare the fully dynamical random-phase approximation results to the quasi-static case in order to emphasize the dynamical correlation effects. Effects of electron-phonon interaction within the two-subband model are also considered.