The presence of internal strain in wurtzite quantum-well (QW) structures may lead to the generation of large polarization fields. These piezoelectric fields cause a spatial separation of the electrons and holes inside the QW to screen the internal fields. A self-consistent calculation of optical gain and the corresponding differential gain is presented in pseudomorphically strained GaN quantum wells as a function of carrier density. Based on the local exchange-correlation potential, electron and hole band structures are obtained by coupling Poisson's equation with an effective-mass Schrodinger equation in the conduction band and an envelope-function (or k/spl middot/p) Hamiltonian in the valence band. Our calculations show that self-consistent calculations including the piezoelectric effects are essential for accurate description of strained wurtzite QW structures.