In the northern hemisphere, O3 levels in the lower atmosphere in industrial countries are rising at a rate of 1 to 2% a year. It is predicted that this trend will continue through the first half of this century. If CO2 continues to increase for the next 50 years at the same rate as it has for the past 100 years, the atmosphere in 2050 will have nearly twice as much CO2 as it did in 1900. Our objective is to measure soybean genetic variation for response to increased CO2 and/or O3 concentrations under field conditions. Twenty soybean cultivars released over the last 50 years were selected for testing in 4 replications in 2004 and 2005. The treatment [CO2] was 550 ppm, 50% above ambient; and treatment [O3] was approximately 25% above ambient. Ambient [O3] was 45 and 50 ppb in 2004 and 2005, respectively. A combination treatment of both elevated CO2 and O3 was also used. Dates of reproductive stages, plant height and lodging, seed yield, and stover yield were recorded. Elevated ozone generally hastened maturity and reduced plant height but the magnitude these changes was not related to yield loss. Yield was reduced approximately 10% in the most susceptible cultivars and less than 2% in the most tolerant entries. Elevated CO2 generally delayed maturity and increased plant height but neither change was predictive of yield increases. Seed yield was not increased in some cultivars but increased over 15% in the most responsive entries. Elevating both CO2 and O3 mitigated the negative impact of O3 alone. In general with the combination treatment, maturity was delayed, plant height was increased, and seed yield was higher than under ambient conditions. However, for some cultivars the yield increase under elevated CO2 and O3 was much greater than under CO2 alone.