Technical Abstract: Switchgrass is an important bioenergy crop with the potential to provide a reliable supply of renewable energy while also removing carbon dioxide from the atmosphere and sequestering it in the soil. We conducted a four-year study to quantify carbon dioxide sequestration during the establishment and early production years of a young switchgrass stand in the northeastern USA. Carbon dioxide uptake was low during the first growing season (2005) due to the immaturity of the crop and to moderate drought conditions which existed that summer. Net carbon dioxide uptake was restricted to a four-month period each year from May through August, whereas the rest of the year experienced a net loss of carbon dioxide to the atmosphere. The amount of carbon dioxide taken up during the summer and the amount lost during the winter were all much greater during the last three years of the study compared with 2004-2005. The amount of harvested biomass increased each year as the stand matured, from 937 kg dry matter/ha for the spring 2006 harvest to 3860 kg/ha in 2007 and 5973 kg/ha in 2008. Averaged over four-years, this field sequestered 1430 kg carbon dioxide/ha/yr (390 kg C). The amount of carbon dioxide sequestered increased each year for the first three years as the amount of carbon dioxide taken up in the summer increased more than the increase in losses due to wintertime respiration and spring biomass removal. However, a small decrease in summer uptake in 2007-2008 compared with the previous year (-7%), combined with a small increase in wintertime loss (+9%) and a large increase in harvested yield (+55%) caused the field to experience a net loss of carbon dioxide in 2007-2008. Switchgrass bioenergy crops in the northeastern USA can function as sinks for atmospheric carbon dioxide for at least the first few years following establishment, in addition to their primary function as a source of renewable energy.