Background measurements of Carbon Monoxide in the extra-tropical Northern Hemisphere during the 1990s showed no clear trends, but significant inter-annual variations. In this study, the measured summertime averaged CO concentrations north of 30¿ N were correlated with area burned by forest fires in North America and Russia. According to a linear regression analysis, 14% of the CO variability in the extra-tropical Northern Hemisphere can be explained by boreal forest area burnt in North America, 53% by area burnt in Russia, and 63% by the combination of both. There are strong indications that the officially reported Russian fire areas are significantly underestimated. By statistically correcting these areas, we can show that, on the average, boreal burning provides a summertime CO source term comparable with all anthropogenic sources in the United States and Europe. ¿ 2001 American Geophysical Union