The sources of odd hydrogen radicals (HOx=OH+HO2) in the upper troposphere are investigated and quantified using a three-dimensional model. While the reaction of O(1D) with water vapor constitutes the single major source of HOx in the lower and middle troposphere, the model calculations suggest that acetone photooxidation represents a large, almost ubiquitous source of HOx in the upper troposphere (around 20--40% of the total primary source in the main aircraft corridors, poleward of 40 ¿N), while the convective injections of peroxides and aldehydes are the dominant sources in the tropics, above the oceans and the continents, respectively. The consequences for ozone photochemical production in the upper troposphere are discussed, in particular, in the perspective of the aircraft impact. The role of acetone on ozone photochemical production is estimated and discussed. It is calculated that the presence of acetone might enhance by about 20% the sensitivity of upper tropospheric ozone to the current aircraft emissions of NOx. ¿ 1999 American Geophysical Union