Ongoing research designed to expand a grid of tree-ring chronologies in the western U.S. that can be used to examine tree growth response to increasing atmospheric CO2 is summarized in this interim report. Current and projected sampling is designed to cover most of the Great Basin and the Southwestern U.S., focusing on long -lived species growing under stressful climatic conditions. Older trees growing in these circumstances provide the best potential for analytical discrimination of climatic and CO2 signals. A descriptive statistical summary of all current data sets is provided and potential directions of the project are discussed.