The optical gain of single quantum well GaAs/GaAlAs laser diodes is studied theoretically. The model uses a nok-selection rule and Fermi statistics to obtain the gain coefficient expression. Gain-current characteristics are then reported and allow comparison of structures with well widths between 50 and 400 Å. Comparison is also made to previous models which use a strictk-selection rule. Then theoretical threshold current densities are calculated for typical single quantum well lasers where the optical confinement is performed using a five-layer slab waveguide. They are shown to be relatively insensitive to the well width as long as Lzis larger than 80 Å. Comparison between two different structures shows that optical confinement plays a critical role for optimizing the threshold Current and should be carefully studied, especially if thek-selection rule is relaxed.