This study examines the effects of customer concentration levels on firm cost structure decisions. Analyzing cost data from a sample of manufacturing firms from 1976 through 2013, we find a negative relationship between customer concentration and cost elasticity whereby firms exhibit lower proportions of variable-to-fixed costs in the presence of higher levels of customer concentration. Additionally, we find that greater customer bargaining power, proxied by supplier industry competition and product market fluidity, leads to lower cost elasticity as customer concentration becomes greater. These results are robust to alternate specifications as well as controlling for endogeneity using a two-stage model. Our results suggest that suppliers respond to customer concentration by pursuing increased mutual dependence and cooperation with customers rather than attempting to reduce the effect of power imbalances within the supplier-customer relationship.