Five cases of psychiatric side effects of cytokines are described. A delirium develops in one patient during combined use of alpha-interferon and interleukine-2. During use of alpha-interferon depressive disorders arise in two patients; a fourth patient develops an anxiety disorder with lability of affect. Furthermore, a depressive episode related to interleukine-12 is described. Problems with assuming causal links between use of drugs and psychiatric disorders are discussed. Theoretical models explaining the induction of psychiatric disorders by cytokines and options for treatment are outlined.