We compared the cerebrospinal fluid (CSF) cytology of 30 acutely psychotic patients at the initial phase of their hospital treatment with that of 46 control individuals with no psychiatric disorder or central nervous system (CNS) disease. The cytological profile of May-Grünwald-Giemsa stained CSF cell slides of the patients was significantly different from that of the control population. The most striking finding was a significantly increased frequency of lymphoid cells showing morphological features of activation/stimulation and a decreased proportion of normal small lymphocytes. Many of the cells with aberrant morphology displayed structural details similar to those of the 'P cells' previously described in the blood of schizophrenic patients. The patients' CSF also contained elevated proportions of monocytes/macrophages, some of which were found in 'rosettes' with activated lymphocytes indicating an increased intercellular adhesion. Possible pathogenic mechanisms behind lymphocyte activation and macrophage dominance in the CSF of acutely ill psychotic patients are discussed.