We have used a three-dimensional chemical transport model to quantify Arctic ozone loss in 2004/2005 and compare it to other winters through 2006. Relative to Arctic stratospheric variability, 2004/05 was a very cold winter with large regions of possible NAT (nitric acid trihydrate) and ice polar stratospheric cloud formation. These areas were the largest during the past 12 years in January and the vortex area was similarly the largest in early March. Accordingly, the model produces strong denitrification, extensive chlorine activation and large chemical ozone loss of up 75% locally and ∼140 DU in the vortex-averaged column, which slightly overestimates that derived from observations. Compared with similar calculations for recent years, 2004/05 compares with 1999/2000 as one of maximum modelled loss. Sensitivity experiments show that small regions of extreme ozone loss, near 100% at some altitudes, could have happened if the winter of 2004/05 was followed by a spring like 1997 with a long-lasting cold polar vortex. Copyright 2007 by the American Geophysical Union.