Nosocomial infections, which are not uncommon in neurosurgical intensive care medicine, may possibly be favoured by an impairment of immunological competence of the patient. In a prospective observational trial, we investigated several parameters of cellular and humoral immunity in 32 patients before and after resection of an intracranial tumour. We quantified the effects of operative procedure, dexamethasone pretreatment, and tumour type. Dexamethasone alone causes an increase of neutrophilic granulocyte count and monocytes, whereas IgG and eosinophilic granulocytes decrease as well as lymphocytes. CD4+ T lymphocytes (T helper cells) and CD8 + T lymphocytes (T cytotoxic/suppressor cells) were more severely affected than B lymphocytes. Dexamethasone and operation in combination act synergistically on T lymphocytes and IgG, while no synergism is obvious in other clinical test parameters. The skin sensitivity reaction was depressed accordingly. With intracerebral tumours (gliomas WHO grades II to IV), levels of T helper cells and eosinophilic granulocytes were lower, and levels of IgM and neutrophilic granulocytes were higher than with benign extracerebral neoplasms. Postoperative nosocomial infections of the lower respiratory tract occurred almost exclusively in patients subject to severe depression of T helper cells.