We present a theoretical analysis of hot-electron bistability in quantum-dot structures. An energy balance approach is used to demonstrate the existence of negative differential conductance due to thermal runaway of the carriers in the dot structure. This runaway is shown to arise from heating of carriers in the quantum dot by incident electrons injected over the barrier of the input constriction. Excellent agreement is obtained with recent experimental evidence of bistability in the current-voltage characteristics of laterally confined quantum dots [J. C. Wu, M. N. Wybourne, C. Berven, S. M. Goodnick, and D. D. Smith, Appl. Phys. Lett. 61, 1 (1992)].