Mindfulness-based treatments for health issues and psychological disorders have increased in popularity and use. A large amount of empirical evidence indicates that these treatments are effective. However, it is possible that these scientific investigations of mindfulness-based treatments have not been conducted with representative samples. In particular, it seems like most participants in these studies are female. In order to explore this, we conducted a systematic review and examined the sample demographics of 117 articles that investigated the effectiveness of mindfulness-based stress reduction or mindfulness-based cognitive therapy via randomized controlled trials. Male participants accounted for less than 29% of the 9820 total participants. These findings suggest that the evidence supporting the effectiveness of mindfulness-based treatments is based on non-representative samples, and future research should work to correct this.