Abstract: Sensitivity information about average lost sales and inventory levels of a lost-sale (s,S) inventory system with respect to the two decision variables s and S is used for "what-if" analysis and optimization of the decision variables. Computer software implements an algorithm that provides sensitivity of average inventory levels as well as average lost sales with respect to changes in s and q(q=S-s) for a lost-sale inventory simulation for any specified demand distribution. The sensitivity information provided is exact when compared to a finite-difference simulation for the case of both discrete and continuous, stationary and non-stationary demands. A finite difference estimate of the sensitivity is obtained by running two simulations where the parameter in question (s or S) differ by a chosen small number, and then calculating the resulting difference in the performance measure of interest, in this case average inventory and lost sales.