We use a 10-band k (.) p Hamiltonian to investigate gain characteristics of 1.3-mum InGaAsN-GaAs 7-nm quantum-well lasers as a function of indium and nitrogen content. The parameters used were obtained by comparison with experimental transition energy data and fitting to measured spontaneous-emission line broadening. We conclude that optimum device performance is obtained by including the minimum amount of nitrogen necessary to prevent strain relaxation at the given well thickness.