The frequency of days with very low total ozone (<225 DU, termed LODs) over northwestern Europe is studied using data from 8 sites from 1952--1963 and 1990--2000. In autumn, the frequency of LODs was similar in both periods. However, in winter, LODs were much more frequent in 1990--2000 than in 1952--1963. The frequency of LODs is a function of both changes in atmospheric dynamics and changes in the mean ozone column. We used a statistical model to address the relative contributions of these two effects. Results show that the changing atmospheric circulation strongly contributed to the increased LOD frequency in winter. The model also suggests a strong effect of a different mean ozone column, but the uncertainty is larger. Because both total ozone and atmospheric dynamics changed little between the two periods in autumn, no large differences in LOD frequency were found during this season.