A stochastic mode-locking model has been developed to study the effect of intrinsic spontaneous noise and carrier shot noise on the phenomenon of active mode-locking in external-cavity semiconductor lasers. This frequency-domain numerical model includes nonlinear carrier dynamics, population pulsation, and self- and cross-saturation of the gain and is sufficiently flexible to account for coupling of multiple harmonic field components. The computer simulation of noise follows Marcuse's approach1 except that, because of active modulation, the Langevin noise strengths are allowed to depend on the time evolution of the gain and photon number. The resulting numerical simulation models the pulse-height probability distribution and the correlation of pulse height deviations under various operating conditions, such as the bias level, modulation strength, modulation type, and frequency detuning. The array of optical pulses are also numerically sampled and averaged to reveal the detailed stable structure of pulses, including possibly the reproducible satellite peaks, as have been evidenced in other experiments.