April-August temperatures are reconstructed from maximum latewood density and ring-width data for a tree-line site in the Canadian Rockies of Alberta close to Athabasca Glacier. The chronology primarily utilizes Picea engelmannii with some Abies lasiocarpa snags. This reconstruction (AD 1073-1983) is the longest densitometrically based summer temperature record from boreal North America. Mean temperatures from 1101-1900 were 0.71°C below the 1961-1990 reference period and 0.33°C below the 1891-1990 mean of the instrumental record. The coldest interval was the first half of the nineteenth century and the major cold intervals. c. 1200-1350, 1690s and the nineteenth century, coincide with local and regional periods of glacier expansion. Warmer periods. c. 1350-1440 and in the present century, are also periods of higher tree-line or tree-line advance at the site. The 1961-1990 reference period is clearly warmer than any equivalent-length period over the last 800 years. This record of summer warmth reinforces evidence of significant warming at several high-altitude and high-latitude sites around the Northern Hemisphere in the late twentieth century. The reconstruction also indicates that glacier advances of the 'Little Ice Age' in the Rockies occurred during a period of fluctuating climatic conditions rather than a long period of sustained cold of several centuries duration.