Mindfulness-based interventions (MBIs) show promising results in both clinical and non-clinical settings. A number of studies indicate that self-reported mindfulness is associated with adaptive psychological functioning and decreased symptom distress. However, there have been no systematic reviews of research on self-reported mindfulness as an outcome of MBIs for clinical and non-clinical samples. It is also unclear to what extent MBIs actually lead to increased and stable self-reported mindfulness. A systematic literature search was conducted to identify studies measuring self-reported mindfulness before and after an MBI. Meta-analytic procedures were used to investigate self-reported mindfulness as an outcome of MBIs. The results show that several questionnaires have been designed to measure mindfulness, and these have been applied to a variety of samples. Although methodological issues preclude definite conclusions, the meta-analysis indicates that MBIs increase self-reported mindfulness. Effect sizes indicate that increases are in the medium range (Hedges’ g = 0.53). However, over half of the studies found no significant effects of MBIs on self-reported mindfulness from pre- to post-intervention. Also, studies of MBIs against active control conditions show no significant advantage for MBIs in increasing self-reported mindfulness. This raises serious questions concerning the validity of the mindfulness questionnaires currently in use. The addition of a full or half day of intensive mindfulness training (retreats) as part of the intervention moderate the effect sizes in positive direction. Implications for future research include the need for analysis of statistical mediation as well as further validation of questionnaires. Comparisons of MBIs to established evidence-based interventions as active control conditions are also called for.