Observations in the 1999--2000 Arctic winter stratosphere show the most severe and extensive denitrification ever observed in the northern hemisphere. Denitrification was inferred from in situ measurements conducted during high-altitude aircraft flights between January and March 2000. Average removal of more than 60% of the reactive nitrogen reservoir (NOy) was observed in air masses throughout the core of the Arctic vortex. Denitrification was observed at altitudes between 16 and 21 km, with the most severe denitrification observed at 20 to 21 km. Nitrified air masses were also observed, primarily at lower altitudes. These results show that denitrification in the Arctic lower stratosphere can approach the severity and extent of that previously observed only in the Antarctic. ¿ 2001 American Geophysical Union