BACKGROUND: Decades of intervention programs that replaced traditional biomass stoves with cleaner-burning technologies have failed to meet the World Health Organization (WHO) interim indoor air quality target of 35-μg m-3 for PM2.5. Many attribute these results to continued use of biomass stoves and poor outdoor air quality, though the relative impacts of these factors have not been empirically quantified. METHODS: We measured 496 days of real-time stove use concurrently with outdoor and indoor air pollution (PM2.5) in 150 rural households in Sichuan, China. The impacts of stove use patterns and outdoor air quality on indoor PM2.5 were quantified. We also estimated the potential avoided cardiovascular mortality in southwestern China associated with transition from traditional to clean fuel stoves using established exposure-response relationships. RESULTS: Mean daily indoor PM2.5 was highest in homes using both wood and clean fuel stoves (122 μg m-3), followed by exclusive use of wood stoves (106 μg m-3) and clean fuel stoves (semi-gasifiers: 65 μg m-3; gas or electric: 55 μg m-3). Wood stoves emitted proportionally higher indoor PM2.5 during ignition, and longer stove use was not associated with higher indoor PM2.5. Only 24% of days with exclusive use of clean fuel stoves met the WHO indoor air quality target, though this fraction rose to 73% after subtracting the outdoor PM2.5 contribution. Reduced PM2.5 exposure through exclusive use of gas or electric stoves was estimated to prevent 48,000 yearly premature deaths in southwestern China, with greater reductions if local outdoor PM2.5 is also reduced. CONCLUSIONS: Clean stove and fuel interventions are not likely to reduce indoor PM2.5 to the WHO target unless their use is exclusive and outdoor air pollution is sufficiently low, but may still offer some cardiovascular benefits.