Rationale: Exposure to biomass smoke has been related to adverse health effects. In Mexico, one household in four still cooks with biomass fuel, but there has been no evaluation of the health impact of reducing indoor air pollution. Objective: To evaluate the health impact of the introduction of an improved biomass stove (Patsari) in Mexican women. Methods: A randomized controlled trial was conducted in the Central Mexican state of Michoacán. Households were randomized to receive the Patsari stove or keep their traditional open fire. A total of 552 women were followed with monthly visits over 10 months to assess stove use, inquire about respiratory and other symptoms and obtain lung function measurements. Statistical analysis was conducted using longitudinal models. Results: Adherence to the intervention was low (50%). Women who reported using the Patsari stove most of the time compared to those using the open fire had significantly lower risk of respiratory symptoms (RR 0.77, 95% CI 0.62-0.95 for cough and RR 0.29, 95% CI 0.11-0.77 for wheezing) adjusted for confounders. Similar results were found for other respiratory symptoms as well as for eye discomfort, headache and back pain. Actual use of the Patsari stove was associated with a lower FEV1 decline (31 mL) compared to the open fire use (62 mL) over 1 year of follow up (p=0.012) for women 20 years and older, adjusting for confounders. Conclusion: Use of the Patsari stove was significantly associated with a reduction of symptoms and of lung function decline comparable to smoking cessation.