This paper examines how the investment horizons of a firm's institutional investors affect its corporate social responsibility (CSR) activities. Using data on U.S. firms' CSR ratings over the 1995-2012 period, we find that longer investment horizons are positively related to CSR. Further, active long-term institutions increase CSR whereas passive long-term institutions have no significant effect. Our results suggest that investors with long-term horizons have more incentives to monitor their firms which leads managers to engage in more vigorous CSR activities. (C) 2019 Elsevier B.V. All rights reserved.