We show that postmortem cerebrospinal fluid (CSF) pleocytosis is a common event. Postmortem cerebrospinal or ventricular fluid was obtained from children and adults. The cells were counted and morphologically characterized using several histochemical markers. Infants exhibit a brisk postmortem CSF pleocytosis. Sudden infant death cases have relatively high CSF counts. Typeable cells are mononuclear and consist of approximately 60%-70% lymphocytes and 20%-40% macrophages. When postmortem duration is greater than 12 h, the cells become vacuolated and cannot be identified. The etiology of these findings requires further study.