Areas affected by forest fires that occurred in 2005 were mapped in the municipalities of Boca do Acre and Lábrea (in the southern part of Brazil’s state of Amazonas) and estimates were made of the loss of biomass and carbon stock and the committed emissions from increased tree mortality due to fire. Fire scars observed on Landsat-5 TM satellite images from 2004 to 2006 were visually interpreted and digitized; over 865.6 km2 of forest affected by fire were mapped, the majority (2.9% of the total forest cover) concentrated along the southern edges of the municipalities, which border on the states of Rondônia and Acre. The greatest loss of biomass due to the increase in tree mortality was indicated by the survey made 4 years after the fires: 4.5 × 106 Mg total (above + below-ground) and 3.7 × 106 Mg (only above-ground). Consequently, 2.2 × 106 Mg C (total) and 1.8 × 106 Mg C (above-ground) of potential carbon emissions were committed from the initial burn of forest biomass and from trees killed by the fire. Emissions occur both through oxidation of dead biomass by decomposition or through combustion in subsequent fire events. Our results indicate that fires can affect extensive tracts of forest and can emit significant amounts of carbon to the atmosphere in periods of drought. Fire plays a significant role as a threat to the biological balance of the forest and causes loss of biomass and emission of greenhouse gases that have critical implications for the future of forests in the Amazon.