In this paper, I examine whether and how customer-base concentration affects supplier firm performance and stock market valuation. To this end, I compile a comprehensive sample of supply chain relationships and introduce a measure, labeled CC, to capture the extent to which a supplier"s customer base is concentrated. In contrast to the conventional view of customer-base concentration as an impediment to firm profitability, I document a positive contemporaneous association between CC and accounting rates of return, which suggests that efficiencies accrue to suppliers with concentrated customer bases. Consistent with a cause-and-effect link between customer-base concentration and firm performance, analysis of intertemporal changes demonstrates that CC increases predict efficiency gains in the form of reduced operating expenses per dollar of sales and enhanced asset utilization. Using stock returns tests, however, Ifind that investors systematically underreact to the implications of changes in customer-base concentration for future firm fundamentals when setting stock prices. Over the thirty-year sample period studied, a zero-investment trading strategy that exploits investors" underreaction yields abnormal returns in the region of 10 percent per year.