A Monte Carlo simulation technique to predict the mean and scatter in fatigue lifetime of certain 2000 and 7000 series aluminum alloys is described. The method is based upon models of surface microcrack nucleation and early growth, which comprise the initiation phase of the fatigue failure process. The experimental basis for the models is discussed, and examples of predicted numbers and lengths of microcracks developed during fatigue of Al 2219-T851 are compared with experiments. Scatter in lifetime and mean lifetime predictions are also discussed. These are obtained from repeated running of the fatigue simulation, for which scatter in lifetime results from the effect of local microscopic variations in alloy microstructure on the time of crack initiation. The association of the method with a "retirement for cause" philosophy is noted.