Three-dimensional model calculations are used together with total ozone observations from the Global Ozone Monitoring Experiment (GOME) and ozone sonde measurements at Ny-¿lesund, Spitsbergen to quantify the chemical ozone loss inside the Arctic polar vortex in winter 1999/2000. GOME shows March 2000 mean Arctic total ozone values of 365 DU, about 100 DU less than the 1980--1989 mean from TOMS data, well reproduced by the model calculations. A comparison of the modeled ozone with a passive ozone tracer and ozone sonde observations at Ny-¿lesund shows that by the end of March 2000 about 2.5 ppmv of ozone are chemically depleted in the lower stratosphere, corresponding to more than 70% ozone loss. At the same time, the inferred loss in total ozone inside or at the edge of the polar vortex is between 90 and 140 DU. The large ongoing loss during March 2000 is likely to be due to widespread denitrification, which maintains high chlorine activation during this period. ¿ 2000 American Geophysical Union