This study investigated the impact of knowledge of results (KR) format on the performance and stress associated with a vigilance task. Also examined was the effect of the interaction of KR-format (Hit-KR, False Alarm-KR, Miss-KR, and a composite of all three formats) with dispositional optimism and pessimism on performance outcome and reported stress state. Hypotheses based upon a theory of feedback intervention were tested. KR regarding correct detections and the composite-KR (KR regarding correct detections, false alarms, and missed signals) enhanced perceptual sensitivity. However, False Alarm-KR and Miss-KR did not. Contrary to expectations based on the theory, performance was unrelated to the traits across all KR conditions. However, the effects of KR-format on self-reports of stress depended on the individual's level of pessimism and optimism. In addition, KR format and personality affected the multiple dimensions of stress state in different ways.