Measurements of wood density of trees in Amazonian forests are necessary to reduce uncertainties in estimates of carbon stocks and of greenhouse-gas emissions from deforestation. Based on samples from 310 trees in 186 species or morpho-species collected near Manaus, Brazil, the present study finds that commonly used wood density estimates found in published lists by species need to be adjusted downward by 5.3%. Taking the average bole density from this study as a standard, wood density overestimations in three prior studies of the central Amazon were found to be 6%, 4% and 0%. Estimates of primary forest biomass and of gross emissions from biomass loss through deforestation will have to be reduced by similar percentages. Considering full disks with bark dried at 103 °C, the mean basic density at breast height in the Central Amazon dense forest was 0.704 ± 0.117 (mean ± 1 standard deviation; n = 310; range, 0.27–0.96); at the top of the bole it was 0.647 ± 0.093 (n = 307; range, 0.26–0.87). The arithmetic mean of the basic density of the trunk—average of the density at breast height and at the top of the bole—was 0.675 ± 0.101 (n = 307; range, 0.27–0.91). The mean basic density of the bole, adjusted for tapering, and using four samples along the bole, was 0.670 ± 0.099 (n = 71; range, 0.38–0.86). The arithmetic mean of the basic density for the same trees was 0.675 ± 0.098 (range, 0.39–0.87). The basic density of central heartwood was 0.766 ± 0.158 (n = 149; range, 0.34–1.06). Significant differences exist between the various published estimates for Amazonian forest biomass and emissions, but we emphasize that revision of density values based on the present study will not reduce these discrepancies; instead, all estimates will shift in parallel to lower levels. Adjustments to biomass and emissions are sufficiently large to be significant for the global carbon balance. For example, an estimate of net committed emissions of 249 × 106 Mg CO2-equivalent C/year for Brazilian Amazonia in the 1990, of which 237 × 106 Mg CO2-equivalent C/year was from net removal of biomass, would be reduced by 14 × 106 Mg CO2-equivalent C/year (5.7%: larger than the 5.3% adjustment to gross emissions because regrowth estimates remain unchanged). Decrease of similar proportions would apply throughout the tropics. For the 1980s, these downward adjustments total 113 × 106 Mg C/year for CO2 effects alone, or approximately 132 Mg CO2-equivalent C/year including trace gases.