Monte Carlo simulation is used to generate a potential break history that is consistent with the performance of the cast iron water main under study. For each break in the potential history, the net present value of expenditures on repairs up to and including the break, plus the net present value of replacement in the following year is calculated. Subtracting the minimum net present value from the net present value for each break derives the economic loss for replacement after each break. The process is repeated over a large number of potential histories to determine which break results in the minimum expected economic loss. On average, this is the optimum break for replacement. The analysis is extended to estimate survivial curves and the value of near perfect information.