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.To evaluate the health impact of the introduction of an improved biomass stove (Patsari; Interdisciplinary Group for Appropriate Rural Technology [GIRA], Patzcuaro, Mexico) in Mexican women.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.Adherence to the intervention was low (50%). Women who reported using the Patsari stove most of the time compared with those using the open fire had significantly lower risk of respiratory symptoms (relative risk [RR], 0.77; 95% confidence interval [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 FEV(1) decline (31 ml) compared with the open fire use (62 ml) over 1 year of follow-up (P = 0.012) for women 20 years of age and older, adjusting for confounders.The use of the Patsari stove was significantly associated with a reduction of symptoms and of lung function decline comparable to smoking cessation.