12 pages, 5 figures.-- PACS nr.: 42.55.Px. Starting from a microscopic model in the free-carrier approximation, we derive an analytical approximation for the optical susceptibility of uniaxially stressed quantum-well lasers at low temperatures by neglecting second-order contributions of the band-mixing phenomenon. The resulting polarization-dependent peak gains, differential peak gains, transparency carrier densities, and linewidth enhancement factors as induced by the uniaxial planar stress are discussed. The authors acknowledge the European RTN network VISTA (Contract No. HPRN-CT-2000-00034) and the COST 288 action. J. D. and G. V. acknowledge the FWO (Fund for Scientific Research—Flanders) for their fellowships and for project support. On the Belgian side this work was supported by the Interuniversity Attraction Pole program (IAP V/18), the Concerted Research Action “Photonics in computing” and the Research Council of the Vrije Universiteit Brussel. S. B. acknowledges financial support from Ministerio de Ciencia y Tecnología (Spain) through project TIC2002-04255-C04-03. Peer reviewed