Improved biomass cookstoves have been promoted as important intermediate technologies to reduce fuelwood consumption and possibly cut household air pollution in low-income countries. This study uses a randomized controlled trial to examine household air pollution reductions from an improved biomass cookstove promoted in rural Ethiopia, the Mirt improved cookstove. This stove is used to bake injera, which is very energy intensive and has a very particular cooking profile. In the overall sample, the Mirt improved cookstove leads to only minor reductions in mean household air pollution (10 percent on average). However, for those who bake injera in their main living areas, the Mirt improved cookstove reduces average mean household air pollution by 64 percent and median household air pollution by 78 percent -- although the resulting household air pollution levels are still many times greater than the World Health Organization's guideline. These large percentage reductions may reflect decreased emissions due to less use of fuelwood, given Mirt's energy-efficient design, and the likelihood that higher-emissions three-stone cooking is moved outside the main living area once a Mirt improved cookstove is installed. Households in the subsample who experience a greater decline in household air pollution tend to be less wealthy and more remotely located and burn less-preferred biomass fuels, like agricultural waste and animal dung, than households that cook in a separate area.