Abstract: This study examined if and how Facebook user-generated comments about the flu vaccine influenced psychological reactance in conjunction with pre-existing flu vaccine attitude. Psychological reactance was further tested as a mediating factor influencing behavioral intention to receive the vaccine. An experimental study was conducted with 264 college students. The results showed that pre-existing attitude positively influenced participants’ intention to obtain the flu vaccine through an increase (a decrease) in psychological reactance when there was inconsistency (consistency) between the pre-existing attitude and valence of comments. Collectively, the results demonstrate that psychological reactance is one possible theoretical mechanism through which social media users’ health behaviors are influenced by user-generated comments.