The etiological role played by human pathogenic herpes viruses for diseases of the central nervous system has been verified (Herpes simplex virus, varicella zoster virus) or is being discussed (cytomegalo virus, Epstein-Barr virus). In this study, the significance of herpes virus infections for surgically treated cerebral tumour patients is examined. In a total of 30 patients with various cerebral tumours, infectious virus from cerebral tumour tissue could be isolated in none of the cases. The mean geometric antibody titres against herpes virus in the patients did not differ from those of comparable healthy controls. In patients with cerebral tumours, however, acute herpes virus infections were serologically identified with a significantly greater frequency than in the control group. They are mainly an expression of endogenous re-infections with these viruses which inhabit latently the human organism. Possible causes of the observed asymptomatically proceeding herpes virus reactivations are discussed.