Individuals undergoing treatment for a symptom like pain expect that the treatment will reduce the pain. Many studies show that healthy volunteers or patients in pain report less pain after inactive treatment, if they believe that active medication has been administrated. The reduction of pain can be partly blocked by systemic administration of naloxone, an opioid antagonist. There is reduced central nervous system activation to painful stimuli in individuals who have been given a placebo and told it is a painkiller. These findings suggest that the expectation of pain relief generates central nervous system opioid activity that inhibits pain transmission to the cerebral cortex. Expectations may thus lead to changes in central nervous system activity that reduces pain. It is proposed that expectations activate a homeostatic system that corrects perturbations to the system via negative feedback. The nocebo effect is the opposite of the placebo effect, and is due to induction of negative emotions. Part of the treatment of many symptoms and diseases is due to autonomic adjustments controlled by the central nervous system. The involvement of emotional processes in placebo effects could have important consequences for interpretation of data from randomized controlled trials.