Nonequilibrium (hot) optical phonon effects on electron runaway from GaAs quantum wires embedded in AlGaAs have been investigated by Monte Carlo technique. We have simulated the carrier runaway kinetics in the0<E<1000 V/cmelectric-field range for a lattice temperature of 30 K. Due to optical phonon mode confinement by GaAs/AlGaAs heterointerfaces, the buildup of generated hot phonons is strongly pronounced in the quantum wires. Even at moderate electron concentrations and electric fields, the accumulation of these phonons may become significant and substantially affect all transport properties in the structure. As a result of reduced hot electron cooling rates in the presence of nonequilibrium optical phonons, the high-energy tail of the carrier distribution function extends above the potential barriers at the quantum wire boundaries. This may eventually lead to significant electron escape from the potential well, even at relatively low electric fields, what significantly affects the performance of such nanoscale systems. ©1997 American Institute of Physics.