Objective: Although there are extensive data on the relationship between personality and stress reactivity in adults, there is little comparable empirical research with adolescents. This study examines the simultaneous relationships between long term functioning (personality, defenses) and observed stress reactivity (affect) in adolescents. Methods: High school students (N = 169; mean age 16; 73 girls) were asked to participate in two conditions of the Stress Induced Speech Task (SIST): Free Association and Stressful Situation. Immature and mature defenses, distress and restraint personality dimensions, and negative and positive affect were examined. Results: Greater reported use of immature defenses was significantly associated with negative affect, whereas greater reported use of mature defenses was significantly associated with greater positive affect. Although personality style was also a significant predictor of negative affect across two out of three conditions, defenses were better overall predictors of affect than were personality dimensions. Gender was also a significant predictor of negative affect, wherein girls reported more negative affect than boys. Discussion: Defenses and personality style predict affective response during a moderately stressful task. Immature defenses and, to a lesser extent, the distress personality dimension predict mobilization of negative affect, whereas mature defenses predict the reporting of positive affect. These results relate to processes central to psychotherapy: defensive responding, personality style, and affective reactivity during the recounting of stressful events.