Satellite measurements have shown a significant decline in ozone at mid-latitudes, and this has only partially been explained by chemical models. Here we show that the large ozone depletion in the Arctic vortex in spring 1997 leads to a dilution of northern mid-latitude ozone after break-up of the vortex of about 3% or more than 1/3 of the observed change in TOMS total ozone at 30--60 ¿N from May 1979 to May 1997. The dilution in spring 1993--96 is crudely estimated to be 2--4%. These results indicate that dilution plays a substantial role in the mid-latitude ozone depletion in spring and summer, when the harmful effects of increases in UV radiation are largest. ¿ 1998 American Geophysical Union