Neuropsychiatric symptoms are commonly related to interferon alpha treatment. The paper summarises the current knowledge about their aetiology, course, and treatment. Interferon alpha is a cytokine with antiviral and antineoplasmatic activity. It is commonly used in the treatment of chronic hepatitis C and B, malignant melanoma, Kaposi sarcoma, renal cancers, and some haematological malignancies. Treatment with interferon alpha is associated with depressive symptoms, cognitive disturbances, chronic fatigue syndrome, dysphoria, anxiety symptoms, anorexia, mania and psychotic states. Up to a half of the patients need psychiatric consultations, 10-25% of them need psychiatric treatment. Neuropsychiatric symptoms are the results of direct affection of CNS by interferon and induced cytokines. They increase hypothalamic-pituitary-adrenal (HPA) activity, alter thyroid function and lead to a behavioural syndrome called 'sickness behaviour'. Moreover interferon induces the activity of 2, 3 indoloamine dioxygenase, the enzyme which converts tryptophan into kynurenine, leads to a reduced level of tryptophan, and thus to a reduced level of central serotonin and to an increased level of neurotoxic kynurenine metabolites. Interferon also affects central opioid receptors and changes dopaminergic and noradrenergic neurotransmission. Serotonin selective reuptake inhibitors (SSRI), other antidepressants i.e. nortriptyline, benzodiazepines, naltrexone, and neuroleptics (for maniac and psychotic states) are used to treat interferon associated psychiatric symptoms. Psychological therapy may also be useful, as well as psychoeducation and behavioural interventions.