INTERNATIONAL differences in the diffusion of the basic oxygen furnace (BOF) have been examined in numerous studies. The reasons for this interest are clear. BOFs are cheaper to build than open hearths, produce steel at lower cost, and better lend themselves to automation and pollution control. Thus, within two decades of the BOF's introduction by a small Austrian steelmaker in 1952, it had supplanted the open hearth as the world's most widely used steelmaking process. A question for research has been whether a slowness to adopt the BOF and other new technologies may have been a factor in the decline of the U.S. steel industry. Widely differing conclusions have been reached. Most analyses, however, have been based on aggregate production statistics. 1 The purpose of this study is to introduce new, disaggregated data to compare the adoption of the BOF by the U.S. and Japanese steel industries. Our data directly compare the number of times the BOF was chosen over the open hearth when firms in the two countries were expanding steelmaking capacity. These data lead to conclusions that differ from those based on aggregate data.