The Five Factor Mindfulness Questionnaire (FFMQ) measures 5 factor-analytically derived mindfulness aspects (Observe, Describe, Non-Judgment, Non-Reactivity, and Acting with Awareness) and is commonly used as an indicator of mindfulness in population surveys and studies of mindfulness-based interventions (MBI). Outside MBI, FFMQ scores are hypothesized to reflect relatively stable human dispositions of importance to psychological health. However, the long-term test-retest reliability of FFMQ scores is virtually untested and it remains unknown whether FFMQ scores predict psychological health after controlling for standardized socioeconomic status classifications. First, we focused on psychometric validation of the FFMQ translated to Danish in a randomly invited healthy and nonmeditating adult community sample (N = 490). Confirmatory factor analyses primarily supported a four-factor construct excluding the Observe facet. The four-factor model showed adequate composite reliability, convergent validity and satisfactory-excellent internal consistency, Cronbach αs = .72-.91. Structural equation modeling revealed that FFMQ Total scores were positively related to income and socioeconomic status but independently predicted psychological distress and mental health scores, respectively, after controlling for age, gender, body mass index, socioeconomic job classification, stressful life events, and social desirability, β = -.24-.29, ps < .001. Second, FFMQ scores showed adequate short-term (two weeks) test-retest reliability among 99 healthy university students, Spearman's ρs ≥ .82. Finally, all FFMQ mean scores showed satisfactory test-retest reliability across a long-term (six months) interval (N = 407), intraclass correlation coefficients ≥.74. We recommend the Danish FFMQ for further use. The Observe facet should be interpreted with caution. Remaining FFMQ facet scores comprise an internally consistent four-dimensional construct reflecting long-term-reliable human dispositions of independent significance for predicting mental health. (PsycINFO Database Record (c) 2018 APA, all rights reserved).