Managers face continuous pressure to meet short-term forecasts and targets, which can potentially impact firms’ investments in customer capital and pricing decisions. Using data on U.S. public companies together with IBES analysts’ forecasts, we find that firms that just meet analysts’ profit forecasts have an average markup growth of 0.8% higher than firms that just miss targets, suggesting opportunistic markup manipulation. To assess the aggregate economic implications of short-termism, we develop and estimate a quantitative firm-heterogeneity model that incorporates short-term frictions and endogenous markups resulting from customer accumulation. In the model, short-termism arises optimally to offset manager’s private incentives, resulting in higher markups and lower customer capital stock. We find that, on average, firms increase markups by 8% due to short-termism, generating $38 millions of additional annual profits. At the macro level, the distortion reduces consumers’ welfare by 4% and lowers the annual total market capitalization by $3.1 trillions on average.