AbstractObjectives Mindfulness-based interventions are part of the third wave of behavioral and cognitive therapies and have resulted in many research studies. Mindfulness can be considered as a state of open attention on the present where people observe theirs thoughts and feelings from a distance without judging them. In this study, we considered mindfulness as a metacognitive competence. This psychotherapeutic approach has proved positives effects in a wide range of mental disorders (recurrent depression, Generalized Anxiety Disorder, bulimia...). However, questions remain about benefits of the third wave of CBT in comparison with behavioral and cognitive waves. Are they of additional benefits and correlated to development of mindfulness? This study aims to test these problems. The following hypotheses were made: Anxious-depressed patients, included in this study, will decrease theirs scores to clinical variables (depression anxiety, affectivity) between pre and post treatment, and this evolution will be linked to the increase in the capacity of mindfulness. Patients and methods Thirty-eight patients (including 10 men and 28 women) from 24 to 63~years (m = 42.89;~σ = 11.89) with a diagnosis of depressive disorder and/or anxious disorder (CIM-10), have followed fifteen group sessions including mindfulness-based intervention (Mindfulness-Based Cognitive Therapy, Acceptance and Commitment Therapy, Unified Protocol of Barlow). In this protocol, the patient's level of mindfulness (MAAS), anxiety (STAI-B), depression (BDI-13), alexithymia (TAS-20) and positive affectivity and negativity affectivity (PANAS) are evaluated at pre- and post treatment (four months of times). Results In pre-test, patients have a low capacity of mindfulness, a moderated depression and anxiety, affectivity negative and they are on the limit alexithymia threshold. After, statistical processing, the results show a positive and significant evolution of protocol on some of the indicators (depression, d = -0.6; anxiety, d = -1.02; positive affectivity, d = -0.47). However, there are not significant evolution for alexithymia and negative affectivity. Furthermore, this research has enabled to show mindfulness is negatively and significantly correlated (P < 0.001) with depression, anxiety and alexithymia, but less with negative affectivity (P < 0.10). Others results indicate that evolution of mindfulness is proportional to evolution of clinical variables (except for anxiety). Conclusions These data don't enable to conclude a causal link, but the results show that improves capacity of mindfulness could improve clinical variables (depression, anxiety, alexithymia). Limits: lack of control group, and absence of follow up test. Mindfulness-based interventions seem to be a useful adjunct treatment. Other studies would be useful to study specific effects of mindfulness.