Chronic treatment with cytokines is associated with the development of mood and cognitive changes that suggests frontal-subcortical cerebral dysfunction. There is large individual variability in the type and severity of specific symptoms that are reported. The CNS effects of cytokines can be disassociated from the effects of chronic disease, other treatments and medications, and psychological responses to illness. The length of treatment and dose are both important factors in the development of mood disturbance. The finding that treatment chronicity is important makes long-term follow-up of patients on cytokine therapy all the more vital. Most adverse effects of cytokines improve with appropriate treatment of symptoms, although dose reduction or cessation of therapy may be necessary in individual cases. Future studies will be needed to better identify at-risk individuals, to compare the efficacy of various interventions, including antidepressants, stimulants, and opiate antagonists, and to assess the feasibility of treating at-risk individuals prophylactically.