We examine the spillover effects that occur within Korean business groups (i.e., chaebols) by focusing on the market reactions of event firms to announcements of credit rating changes. We find that there are positive spillovers (caused by positive market reactions) and negative spillovers (caused by negative market reactions) that are driven by the market reactions of event firms. Our analyses indicate that negative spillovers are more dominant than positive spillovers. Moreover, a spillover that is driven by a leading firm within a business group has stronger effects on other firms in the group than a spillover that is driven by a non-leading firm. This suggests that the market evaluation of a business group is conducted more on the basis of a leading firm than a non-leading firm within a group. Finally, we show that the spillover effects that are analyzed in our study are more noticeable when the business relationship between an event firm and other affiliated firms is closer. (JEL G14, G24)