Cell-mediated and humoral immunity was studied in 74 patients with acute cerebral vascular disease. During the first two days after the onset of disease marked changes of cell-mediated immunity were observed, manifested as a decrease in total lymphocyte count in the peripheral blood, decrease in number of T lymphocytes, depression in lymphocyte blastogenesis and production of the migration inhibition factor, and a delayed-type skin reactivity. The changes were most evident in patients with severe lesions of brain tissue resulting from primary cerebral haemorrhage and cerebral infarction with fatal outcome. In the group of patients with cerebral infarction with improvement of neurological symptoms the immunological changes were not so pronounced as in the two above-mentioned groups, the smallest changes being found in patients with subarachnoid haemorrhage. We suppose that the depression in the immunne function was caused by severe stress during the course of disease. Impairment of the immune function may increase susceptibility to infection. The humoral immune response was not so evidently changed, and the observed increase of IgA in the sera was probably present before the stroke. In cases with good clinical course some improvement in the immunological parameters was observed, but full recovery did not occur until 3 weeks after the onset of disease.