ObjectivesMindfulness‐based stress reduction (MBSR) promotes numerous psychological benefits, but few studies have identified for whom MBSR is most effective. The current study tested the hypothesis that lower baseline mindfulness invites more “room to grow” and, thus, predicts greater improvement during MBSR. Method We examined three facets of mindfulness (awareness, acceptance, decentering) among 131 MBSR participants prior to enrollment, to test the hypothesis that lower baseline mindfulness predicts greater improvements in perceived stress, positive affect (PA), and negative affect (NA) following MBSR. Results Lower acceptance and decentering predicted greater decreases in perceived stress. Higher awareness, acceptance, and decentering predicted greater increases in PA. Higher awareness predicted greater reductions in NA. Lower decentering predicted greater reductions in NA. Conclusion Findings partly supported the hypothesis that lower baseline mindfulness predicts greater improvement following MBSR and emphasize the importance of assessing multiple mindfulness facets given their unique, contrasting relations to outcomes.