The hemlock woolly adelgid is a nonnative pest responsible for extensive mortality and decline of hemlock trees in the eastern United States. Its range currently includes counties in 16 states from the Smoky Mountains to southern Maine, but cold winter temperatures may be a limiting factor in the insect's northward spread. We assessed hemlock woolly adelgid mortality at 36 sites in 6 states after the winter of 2003, and at 35 sites in 10 states after the winter of 2004, the coldest winters recorded in the past 9 years in the Northeast. Percent mortality of adelgids was compared with latitude, longitude, elevation and recorded temperatures at each site.