This study investigates whether institutional investors with large ownership (institutional blockholders) have informational advantages over other investors. The sample of this study is constructed from institutional investors' mandatory filings on block shareholding of greater than 5% of a firm's shares. Institutional blockholders are classified into active institutions if they express an intention to engage in a firm's management as their purpose of investment and into passive institutions otherwise. To verify the information content of block ownership, this study investigates the short- and long-term market reaction to announcements of institutional investors' block ownership. Main findings are summarized as follows. First, significant cumulative abnormal returns (CARs) are observed around the announcements. The CARs are much higher for active institutions than passive institutions. Second, the stocks owned by passive asset management companies significantly outperform both the market and the benchmark portfolio in the long-term. In contrast, the stocks owned by active private investment companies significantly underperform the benchmark portfolio. Overall, the results of this study suggest that passive asset management companies have superior skills in selecting undervalued stocks and they use long-lived information when they make large investment. In contrast, the inconsistent results on short- and long-term performance of stocks owned by active private investment companies imply that private investment companies' activism are not effective enough to meet the initial expectations of the market.