Technical Abstract: Higher leaf transpiration efficiency (TE) without lower photosynthesis has been identified in some varieties of corn in field tests, and could be a useful trait to improve yield under dry conditions without sacrificing yield under favorable conditions. However, because the carbon dioxide concentration in the atmosphere continues to increase rapidly, and stomatal conductance and photosynthesis sometimes acclimate to elevated carbon dioxide concentrations, it is not certain that varietal differences in leaf TE occurring at the current ambient carbon dioxide concentration would still occur at elevated carbon dioxide. Two varieties of sweet corn found in previous field studies to have high leaf TE and two with low TE were grown for two seasons in plots equipped with free-air carbon dioxide enrichment systems to determine whether development and measurement at elevated carbon dioxide would alter the ranking of the varieties in leaf TE. Measurements were also made at ambient carbon dioxide two hours and five days after termination of the carbon dioxide application to separate effects of growth and measurement carbon dioxide concentration on the comparative TE of the varieties. The results indicated that growth at 1.4 times the current carbon dioxide concentration resulted in one variety having about 25% higher leaf TE than the other three varieties, which did not differ significantly from each other. The changed ranking of one variety was a result of growth at elevated carbon dioxide, not the elevated measurement concentration. Thus, in one variety, the high leaf TE trait was stable across growth carbon dioxide conditions, but in another variety, the high TE trait was no longer evident when the plants were grown at elevated carbon dioxide. Based on these results, screening of corn genotypes for high leaf TE at projected future carbon dioxide concentrations may be more efficiently accomplished at those concentrations.