The authors conducted three studies to construct and examine the psychometric properties of a 27-item version of the Mood and Anxiety Symptom Questionnaire-90 (MASQ-90; Watson & Clark, 1991a). The Anxiety Depression Distress Inventory-27 (ADDI-27) contains three empirically derived scales: Positive Affect, Somatic Anxiety, and General Distress, which are relevant dimensions of the tripartite model of affect. Each scale is composed of nine items, and the estimate of scale reliability for each scale score was ≥.80 across the three studies. Results of exploratory and confirmatory factor analyses provided adequate support for a 3-factor model. Additional estimates of concurrent validity documented the ADDI-27 scales' convergent and discriminant validity. We also identified three construct relevant correlates for each scale score. Overall, the ADDI-27 appears to be a content valid, reliable, and multidimensional measure of the tripartite model of affect. © 2011 Wiley Periodicals, Inc. J Clin Psychol 67:1–18, 2011.