Lateral mode stability in index guided semiconductor lasers is discussed. As is well known, the fundamental mode self-focuses at high power due to perturbations in the refractive index profile produced by spatial hole burning. However, the lateral waveguide is not strengthened for all modes. The first order mode, for example, is actually slightly defocused. In strong waveguides the gain profile perturbation due to hole burning is the dominant cause of the first-order mode reaching threshold. In weak wave-guides, however, the defocusing effect can compete with the gain profile perturbation effect leading to stable fundamental mode operation at high output powers.