The environmental impact of using recombinant bovine somatotropin (rbST) in dairy production was examined on an individual cow, industry-scale adoption, and overall production system basis. An average 2006 U.S. milk yield of 28.9 kg per day was used, with a daily response to rbST supplementation of 4.5 kg per cow. Rations were formulated and both resource inputs (feedstuffs, fertilizers, and fuels) and waste outputs (nutrient excretion and greenhouse gas emissions) calculated. The wider environmental impact of production systems was assessed via acidification (AP), eutrophication (EP), and global warming (GWP) potentials. From a producer perspective, rbST supplementation improved individual cow production, with reductions in nutrient input and waste output per unit of milk produced. From an industry perspective, supplementing one million cows with rbST reduced feedstuff and water use, cropland area, N and P excretion, greenhouse gas emissions, and fossil fuel use compared with an equivalent milk production from unsupplemented cows. Meeting future U.S. milk requirements from cows supplemented with rbST conferred the lowest AP, EP, and GWP, with intermediate values for conventional management and the highest environmental impact resulting from organic production. Overall, rbST appears to represent a valuable management tool for use in dairy production to improve productive efficiency and to have less negative effects on the environment than conventional dairying.carbon footprint ͉ environment ͉ greenhouse gas ͉ sustainability ͉ productive efficiency