PURPOSE OF REVIEW An overview is provided that discusses brain dysfunctions in intensive care unit patients in the context of psychoneuroimmunology. Recent findings are discussed within the context of the relationship between behavioural aspects and immune functions. The concept of 'sickness behaviour' is introduced to understand the more differential aspects of stress, fatigue and mood. RECENT FINDINGS Psychoneuroimmunology is a relatively new field of study that investigates interactions between behaviour and the immune system, mediated by the nervous system. 'Sickness behaviour' describes behavioural changes or reactions of an (infected) organism to cope with illness. Sickness behaviour should be seen as an adaptation to an infection that also influences the process of recovery or further illness. SUMMARY There are plausible explanations of how psychological factors (such as aspects of mood) might influence immunity and immune system-mediated disease. There is substantial evidence that psychological factors can influence both cellular and humoral indicators of immune status and function. There are consistent and convincing reports of links between stress and disease onset and progression.