Cytokines are a class of glycoproteins whose properties include immunomodulation. A well-known cytokine is interferon, that is used in the treatment of (hemato-)oncological disorders, multiple sclerosis and some forms of viral hepatitis. Currently, treatment with other cytokines is restricted to clinical trials. Cytokines may produce psychiatric side effects. The psychiatric side effects of interferon are well documented. Especially organic psychosyndromes and mood disorders are related to use of interferon and possibly to the use of other cytokines.