We model nonequilibrium transport in a GaAs-Al x Ga 1−x As quantum-well structure using an ensemble Monte Carlo simulation of the full multisubband system in which we include electron-electron (e-e) scattering explicitly into the calculation. The e-e scattering cross section is calculated using the Born approximation and introduced into the transient Monte Carlo simulation via a self-scattering technique. This interaction is found to be especially effective in transferring energy between different subbands, thus thermalizing the carriers within a picosecond. The model described herein is applied to the study of laser-excited carriers in a quantum-well system and to the response of such a system to high parallel electric fields. In the case of laser excitation, e-e interaction may dominate the initial evolution, reducing the cascade of carriers via optical-phonon emission.