In daily life, not stressful events themselves but their sustained cognitive representation is likely to cause prolonged physiological activity, which is believed to lead to a pathogenic state and finally somatic disease. The typically human ability to make cognitive representations of past stressful events (rumination) or feared events in the future (worry) is called perseverative cognition (PC). PC is associated with increased activity in various bodily systems, and there is emerging evidence that it mediates the prolonged effects of stressors on physiology and on disease. Yet, there are strong reasons to believe that people may not be aware of the greater part of their stress-related cognitive processes, while several studies suggest that these processes may still cause increased physiological activity, during sleep as well as during waking. This may imply that unconscious PC is an even more important source of prolonged stress-related activity than conscious PC. Thus, 'unconscious stress' research has the potential to become a new important area and may yield new important markers of chronic stress.