Traditional self-report measures of psychopathology may be influenced by a variety of recall biases. Ecological momentary assessment (EMA) reduces these biases by assessing individuals' experiences as they occur in their natural environments. This study examines the discrepancy between trait questionnaire, retrospective report, and EMA measures of affective instability in psychiatric outpatients either with a borderline personality diagnosis (n = 58) or with a current episode of major depressive disorder or dysthymia (n = 42). The authors examined the agreement of 3 trait measures of affective instability--the Affective Instability subscale of the Personality Assessment Inventory-Borderline Features scale (L. C. Morey, 1991), the Affect Intensity Measure (R. J. Larsen, E. Diener, & R. Emmons, 1986), and the Affect Lability Scales (P. D. Harvey, B. R. Greenberg, & M. R. Serper, 1989)--and 1 retrospective mood recall task with EMA indices of mood and mood instability. Results indicate only modest to moderate agreement between momentary and questionnaire assessments of trait affective instability; agreement between recalled mood changes and EMA indices was poor. Implications for clinical research and practice and possible applications of EMA methodology are discussed. (Contains 7 tables and 2 footnotes.)