The problem of the density-dependent homogeneous broadening of microcavity polaritons due to exciton-exciton Coulomb interaction is addressed using the self-consistent Born approximation. The theoretical results predict that, for resonant pumping of the lower polariton, the dependence of its broadening on the density of excitations has a threshold behavior. Below the threshold density, scattering is strongly inhibited because it involves only the small region of k space where the polariton effective mass is very light. Above threshold, the region of k space with excitonlike dispersion becomes allowed, giving the dominant contribution to the collision broadening. [S0163-1829(98)52740-3].