Today 2.6 billion people in developing countries rely on biomass as primary cooking fuel, with profound negative implications for their well-being. Improved biomass cooking stoves are alleged to counteract these adverse effects. This paper evaluates take-up and impacts of low-cost improved stoves through a randomized controlled trial. The randomized stove is primarily designed to curb firewood consumption but not smoke emissions. Nonetheless, we find considerable effects not only on firewood consumption, but also on smoke exposure and smoke-related disease symptoms - induced by behavioural changes at the intensive margin affecting outside cooking and cooking time due to the new stove.