Institutional investors are obligated to actively exercise their shareholder rights to perform fiduciary duties as trustees. Korean institutional investors, however, do not perform these duties diligently because they indirectly invest in entrusted assets and are reluctant to exercise shareholder rights out of concerns over investing companies, political affairs, and institutional constraints. Foreign investors, in contrast, are more likely to exercise their shareholder rights because they are free from these concerns. This paper investigates the role of domestic and foreign institutional investors as corporate monitors using a daily mechanism rather than voting rights, which are exercised only on specific proposals at general shareholder meetings. We analyze how the agency costs and value of target firms are affected by the sale of a portion of their stocks by domestic and foreign institutional investors. Our sample comprises all institutional investors' disclosures of an equity ownership change to abide by the “5% rule” in the period from April 2005 to December 2012. Under the 5% rule governing large share acquisitions of publicly held companies, a person or institution who holds 5% or more of the equity securities in a target company or thereafter changes its holdings by 1% or more must file a report with the Financial Supervisory Commission and Korea Exchange (KRX) within five days of the transaction or change date. We exclude from the sample announcements made by a target company between the trading date and announcement date of stock sales because the firm announcement effect and institutional investor disposal effect are likely to be intertwined. We also exclude disclosures that generate a positive market response because only unfavorable announcements are followed by monitoring-motivated stock sales. If a target firm makes an announcement before an institutional investor's announcement of share disposal, we include that announcement only if it shows a statistically significant negative market response at the 10% level, as measured by the cumulative abnormal returns (CAR) around the stock sale disclosure date. CAR is calculated by summing all market-adjusted abnormal returns around the target firm's announcement. If a target firm makes no announcement within five days of institutional investors' announcement of a stock sale, the announcement is included in the final sample to ensure that we do not rule out the disposal of a portion of shares that is too small to attract an announcement obligation by institutional investors to punish management seen to be destroying shareholder value. A sale is regarded as a monitoring-motivated sale when the CAR around the announcement date of an institutional investor stock sale is positive at the 10% significance level. We expect that institutional investors' sale of a portion of their stake reduces a target firm's agency costs and increases corporate value if it has a “kick-in-the-pants” motivation and that strategic investors are more likely to reprimand and exert influence on management than financial investors. However, the opposite outcome is also possible if management subordinates shareholders' passive stock sales to the more active exercise of shareholder rights. In multiple regression analysis, the dependent variables are agency costs measured by the ratio of operating expenses to sales, which tend to decrease when managers' perks are restrained, and the asset turnover ratio, which tends to increase when assets are used more efficiently. Another dependent variable is firm value measured by (industry-adjusted) Tobin's Q. The explanatory variables are a monitoring dummy indicating a positive market response, an institutional investor intention-to-hold stock dummy indicating a strategic investment, an unfavorable announcement-by-the-target-firm dummy, a foreign investor dummy, and a foreign investor intention-to-hold stock dummy. Two-stage least-squares regression analysis is performed with one year lagged data to consider the possible endogeneity of the monitoring dummy representing monitoring motivation. We find that an institutional investor's sale of a portion of its stock is more likely to reduce operating expenses relative to sales when an unfavorable disclosure by the target company generates a positive market response to the institutional investor's announcement of that sale and when foreign investors are labeled as strategic investors. Our results also show that stock sales by institutional investors improve firm value. For foreign investors, firm value is improved only when they report the purpose of their stock holdings to be management participation. Strategic institutional investors tend to have a negative effect on firm value except in the case of kick-in-the-pants-motivated sales, which suggests that we should encourage strategic foreign investors to improve the long-term performance of firms. In sum, institutional and foreign investors monitor firms through the sale of a portion of their shares, in addition to registering a “no” vote on firms' proposals in shareholder meetings and engaging in face-to-face talks with management.