In the last 20 years, an increasing number of studies have been dedicated to research on mindfulness and the use of mindfulness training as a clinical intervention for diverse physical and mental disorders. Mindfulness refers to an awareness that emerges by paying attention to purpose and to the present moment and non-judgmentally focusing on the unfolding of one’s immediate experience [1, 2]. Mindfulness is a skill that can be taught using several uniquely designed techniques .
Mindfulness-based therapies have been demonstrated to be effective for the treatment of many disorders, including chronic pain conditions [4–6]. The mechanisms underlying the effects that mindfulness training has on health are diverse and include increased attention control, increased awareness of inner experiences, increased emotional regulation, and changes in the concept of self or in body awareness .
Mindfulness training in the treatment of fibromyalgia (FM) has been shown to decrease pain symptoms and to improve overall quality of life; as such, mindfulness training is considered a promising supplement to current interventions [4, 8–10]. Despite these findings, there is still a lack of understanding of the mechanisms that underlie the mitigating effects of mindfulness on pain symptoms. Research studies on such mitigating effects suggest that mindfulness alters the contextual evaluation of pain , reduces pain catastrophising and pain sensitivity , reduces psychopathological symptoms [11, 12], and alters pain-related anxiety . These results have not been contradicted in the three years since their discovery .
Recent findings suggest that pain acceptance, which is promoted by mindfulness interventions, improves functioning and life quality. However, there is still a lack of reliable and valid instruments to assess relevant processes in such interventions . It is assumed that if mindfulness is a learned skill, then a measure of mindfulness should demonstrate both incremental validity  and sensitivity to change. Furthermore, the expected changes (for example, improvement in quality of life or decrease in symptoms) should be directly related to changes in mindfulness.
There are several questionnaires that measure mindfulness, with the two the most commonly used being the Five-Facets Mindfulness Questionnaire (FFMQ)  and the Mindful Attention Awareness Scale (MAAS) . The FFMQ is considered one of the most complete questionnaire because it measures five component skills of mindfulness: observing, describing, acting with awareness, nonjudging of inner experience and nonreactivity to inner experience. However, the MAAS is the most popular scale measuring mindfulness, with over 350 citations in the Web of Science . The MAAS has shown theoretically consistent relationships to brain activity , treatment outcome in mindfulness-based interventions (MBIs)  and mediation of targeted MBI outcomes .
The authors that developed the MAAS define mindfulness as “the presence or absence of attention to, and awareness of, what is occurring in the present moment”. The MAAS is a 15-item scale developed to measure the frequency of mindful states in daily life. Translated variants of this scale have been validated in several languages, including Spanish , Chinese , Swedish , Turkish  and French .
The original Spanish scale was validated and developed in a sample of Spanish non-clinical participants. . Within the clinical population, the MAAS has been validated only in a sample of cancer patients . The validation of scales in specific clinical samples is important for research on mindfulness due to the recognised need for using valid measures in the assessment of interventions. In a recent study, the Five Facets of Mindfulness Inventory was validated in a sample of patients diagnosed with fibromyalgia. The results from this study showed that the data taken from the patient sample had a similar factorial structure to data taken from a healthy sample . The purpose of the present study is to examine the psychometric properties of the Spanish version of the MAAS in a sample of patients diagnosed with fibromyalgia.