We give a detailed theoretical study of the optical gain for compressive and tensile-strained GaInAs/GaInAlAs quantum-well lasers, including valence-band mixing. We demonstrate that with strained quantum wells, lower thresholds and higher differential gains are expected. We also show that these characteristics are even better than those of the most usual GaInAs/GaInAsP system. The good experimental results already obtained tend to confirm the theoretical expectations. (C) 1995 American Institute of Physics.