The cytology of the cerebrospinal fluid (CSF) (obtained by suboccipital puncture) from patients after cardiac arrest was investigated by means of the cytocentrifugation method.In patients with unsuccessful resuscitation an increased number of monocytes and macrophages was found immediately after abandoned resuscitation.A pronounced increase in the number of granulocytes, monocytes, and macrophages was found in patients with poor neurologic restitution. the increase in the number of macrophages was seen immediately after cardiac arrest. the granulocyte increase was most pronounced after 24 h.The CSF from patients with full neurologic restitution after cardiac arrest showed small numbers of lymphocytes and granulocytes and an equal amount of monocytes and macrophages.The findings indicate that monocytes/macrophages appear in the cisternal fluid shortly after anoxic/ischemic damage to the brain and that the increase of macrophages and later granulocytes probably is a result of damage to brain tissue.