Tree-ring evidence from 97 sites over North America and Europe are used to develop a chronology of widespread cool summers since 1600. Averaging annual maximum-density information from the chronologies provides an index that correlates with Northern Hemisphere summer temperatures to 0.44 over 1851-1975. A number of the extreme low density years occur in both North America and Europe, suggesting a common response to high-frequency forcing. The analysis was repeated with ring-width information but the results are markedly less coherent. Of the five common extreme low-density years (1601, 1641, 1669, 1699 and 1912) four are known to be coincident with the year or year following major volcanic eruptions. Other extreme low density indices in the continental series are also dearly coincident with known volcanoes. ?he density indices clearly demonstrate great potential for quantifying the climate effectiveness and perhaps establishing precise calendar dates of large explosive volcanoes.