In a seminal article, Chinitz (1961) considered the effects of industry size, structure, and economic diversification on the performance of firms and regional economies. His inquiry suggested a related but conceptually distinct issue: how does the extent to which a regional industry or industrial sector is concentrated in a small number of firms affect the local performance of that industry? The question has not been addressed systematically in empirical research other than case studies, principally because accurately measuring regional concentration requires firm-level information. This exploratory study uses confidential plant-level data to gauge concentration in manufacturing industries at the regional scale across the continental United States, to explore changes over time in geographic patterns of concentration, and to investigate associations between regional industrial structure concentration and changes in employment. The implications for understanding the impacts of regional industrial structure on economic development processes are discussed.