This paper compares EPA’s ex ante cost analysis of the large municipal waste combustor (MWC) rule to an ex post assessment of its cost. For our analysis of the MWC rule, we use plant-level data from the U.S. Department of Energy annual survey of pollution abatement expenditures by steam-electric power plants and from a survey of municipal waste combustor plants compiled by Government Advisory Associates. We find the ex post capital expenditures for nitrogen oxides control systems are typically lower than the EPA ex ante estimates, while ex post capital expenditures for mercury control systems tend to be higher than the EPA ex ante estimates. Finally, the comparison of ex post capital expenditures for particulate and sulfur dioxide control to ex ante capital costs are mixed. While a few plants are outliers when comparing the ratio of ex post capital costs to ex ante capital, the mean of the comparison across plants is near unity.