Three-dimensional model calculations suggest that the world's fleet of subsonic aircraft has enhanced the abundance of nitrogen oxides in the upper troposphere by up to 20–35% and has produced a significant increase in the ozone concentration in this region of the atmosphere (4% in summer and 1% in winter). In year 2050, on the basis of current scenarios for growth in aviation, the concentration of NO_x at 10 km could increase by 30–60% at midlatitudes, and the concentration of ozone could be enhanced by 7% and 2% in summer and winter, respectively (relative to a situation without aircraft effects). The perturbation is not limited to the flight corridors but affects the entire northern hemisphere. The magnitude (and even the sign) of the ozone change depends on the level of background atmospheric NO_x and hence on NO_x sources (lightning, intrusion from the stratosphere, and convective transport from the polluted boundary layer) and sinks which are poorly quantified in this region of the atmosphere. On the basis of our model estimates, 20% of the NO_x found at 10 km (midlatitudes) is produced by aircraft engines, 25% originates from the surface (combustion and soils), and approximately 50% is produced by lightning. For a lightning source enhanced in the model by a factor of 2, the increase in NO_x and ozone at 10 km due to aircraft emissions, is reduced by a factor of 2. The magnitude of aircraft perturbations in NO_x is considerably smaller than the uncertainties in other NO_x sources.