Improved biomass cookstoves have the potential to reduce pollutant emissions and thereby reduce pollution exposure among populations in developing countries who cook daily with biomass fuels. However, evaluation of such interventions has been very limited. This article presents results from a study carried out in 30 households in rural Guatemala. Twenty-four hour PM3.5 concentrations were compared over 8 months for three fuel/cookstove conditions (n = 10 households for each condition): a traditional open fire cookstove, an improved cookstove called the plancha mejorada, and a liquefied petroleum gas (LPG) stove/open fire combination. Twenty-four hour geometric mean PM3.5 concentrations were 1560 micrograms/m3 (n = 58; 95% C.I. 1310, 1850), 280 micrograms/m3 (n = 59; 95% C.I. 240-320), and 850 micrograms/m3 (n = 60; 95% C.I. 680-1050) for the open fire, plancha, and LPG/open fire combination, respectively. A generalized estimating equation model showed a 45% reduction in PM3.5 concentrations for the LPG/open fire combination as compared to the open fire alone. The difference approached significance (p < 0.0737). The plancha showed an 85% reduction in PM3.5 concentrations as compared to the open fire (p < 0.0001). An analysis of the interaction of time with stove type showed that the temporal trend in pollution did not significantly differ among the three stove types. The reduced PM3.5 concentrations were maintained over time. Season did not affect pollutant concentrations. Of the two interventions, the plancha appears to offer the best prospects for achieving substantial reductions in indoor air pollution levels, although issues of cost and stove maintenance remain to be addressed.