International comparison of energy intensity is receiving increased scientific and political attention. International comparisons may help in identifying the potentials for energy intensity reduction. Knowledge of these reduction potentials can be used as the basis of national policies to reduce energy intensity, and also for designing international actions to curb the threats of climate change. Although we showed in a previous paper that indicators based on physical production data are well suited for international comparison of energy intensity, many potential problems exist regarding the availability and quality of energy and production data needed for such indicators. We studied these data problems for physical energy intensity indicators for the iron and steel industry. We started by comparing the energy data from four international data sources. It turned out that quite some mistakes are made in the reported energy data, which makes reliable international comparison of countries difficult. The available production data, and the methodology of using physical production indicators, turned out to be less problematic. We found that the accuracy of physical energy intensity indicators is, to a large extent, determined by the accuracy of the energy consumption data used. We recommend that energy analysts be careful in using energy data for international comparisons. We also recommend that national and international statistical organizations put more effort in assuring the quality of the energy consumption data in their publications.