International comparisons of market structure are complicated by a lack of comparable data. Although the U.S. Department of Commerce reports measures of industry concentration, they do not verify either the control of subsidiary firms or the possible multinational nature of their ownership nor are consistently based on sales. Other counties produce similar reports, but these studies are generally not comparable to U.S. values due, in part, to incompatible sector definitions. In addition, few government- sponsored studies provide firm-level detail or timely information. Furthermore, given the widespread multinational nature of many larger firms, an international analysis of ownership is necessary. This paper addresses the issues encountered in the construction of international market data from traditional financial report data and provides a set of methods for the comparison of measures of market concentration and industry diversity across countries. Using 1991 financial data, a firm level data set is constructed and used to compute comparable measures of market concentration and industry diversity for the U.S. and E.C. food processing industries.