In a seminal article, Benjamin Chinitz (1961) raises the question of the effects that industry size, structure, and economic diversification may have on firm performance and regional economies. His line of inquiry suggests a related but conceptually distinct issue: how does the extent to which a industry is regionally dominated—concentrated locally in a single or small number of firms—impact the local performance of that industry? This question has received little attention, principally because accurately measuring industrial concentration at the regional scale requires firm-level information. This paper makes use of confidential plant- and firm-level manufacturing data to explore patterns of industrial concentration in the United States at the regional scale. Regional analogues of concentration ratios and other measures commonly used in the aspatial industrial organization literature indicate the extent to which manufacturing activity is concentrated in a small number of firms. Both the manufacturing sector as a whole and major manufacturing industry sectors are examined in order to determine the extent of industrial concentration in the continental United States, to explore changes over time in geographic patterns of concentration, and to investigate associations between industrial concentration and employment growth at the regional scale. Implications for understanding regional growth and for devising regional economic development policy are discussed.