The concept of well-being which focuses on positive emotions has received increased research attention. However, a consensus definition of this term is lacking. The Well-Being Index scale (WHO-5) is a generic, self-report scale that contains five Likert-type items to evaluate psychological well-being. This construct may provide a relevant outcome in bipolar disorder (BD) research and care beyond the rating of mood symptoms. Thus, in the current study, the psychometric properties of the WHO-5 Spanish version were assessed in a sample of euthymic patients with BD.Patients with BD- I and BD-II and healthy controls completed the Well-Being Index (WHO-5) together with an assessment of depressive (Hamilton Depression Rating Scale-17; HAM-D) and manic symptoms (Young Mania Rating Scale; YMRS); and a measure of psychosocial functioning (Functioning Assessment Short Test; FAST). Internal consistency reliability was measured through Cronbach's alpha. Test-retest reliability was calculated comparing the WHO-5 total score at baseline and after 10 days of the first administration. To assess the structure of the scale, a principal component analysis (PCA) was carried out. Correlations between the WHO-5, HAM-D, YMRS and FAST were calculated. Finally, a t-test for independent samples was applied to compare the WHO-5 total score in the patient and control groups.A total of 104 patients with BD and 40 healthy controls were included in this study. A Chronbach's alpha of 0.83 indicated acceptable internal consistency. A paired sample t-test revealed no significant differences between WHO-5 total score at baseline and at follow-up (tn = - 0.72; df = 15; p = 0.48). The PCA provided a single factor solution that accounted for 59.74% of the variation in WHO-5. Test-retest reliability was high (r = 0.83; p < 0.001). Moderate negative correlations were observed between the WHO-5 total score, the FAST (r = - 0.46.; p < 0.001) and the HAM-D (r = - 0.68; p < 0.001), but not with the YMRS (r = - 0.07; p = 0.42). Finally, significant differences were found when comparing the WHO-5 total score between patient and healthy controls (t = 5.1; df = 147; p < 0.001).some limitations include the lack of a comparator scale to test for validity construct and the small sample size in the test-retest reliability CONCLUSIONS: The WHO-5 shows an acceptable reliability index and measures a unitary construct in a Spanish population of euthymic patients with BD.Copyright © 2017 Elsevier B.V. All rights reserved.