Energy use and carbon dioxide emissions for the Mexican iron and steel industry are analyzed from 1970 to 1996. To assess the trends in energy use and carbon dioxide emissions, we used a decomposition analysis based on physical indicators to decompose the intra-sectoral structural changes and efficiency improvements. We used a structure/efficiency analysis for international comparisons, considering industrial structure and the best available technology. This study shows that steel production growth drove up primary energy use by 211% between 1970 and 1996, while structural changes (production and process mix) decreased primary energy use by 12% and energy efficiency changes drove down energy use by 51%. In addition, carbon dioxide emissions would have increased by 9% if the primary fuel mix had remained constant at 1970 levels.