We document that stocks of companies belonging to the same business group co-move with each other more than do stocks in the same industry. The within-group correlation in excess of the withinindustry correlation has become more pronounced over time, especially following the 1998 Asian crisis. Fundamental factors, such as related-party transactions, cross-holdings of equity stakes, or earnings correlation do not explain the increased stock return commonality within business groups. Rather, trading in the same-business group stocks is highly correlated more so during the post-crisis period. Such pattern is robust to using daily, weekly, and biweekly returns. Overall, our results are consistent with the notion that greater emphasis on corporate governance post the crisis, particularly on business group-related issues, leads investors to categorize stocks in a given business group and treat them as one entity.