The correlation existing in several human malignancies between lymphocytic infiltration and prolonged survival prompted this study. Two hundred selected patients who were operated on for glioblastoma were reviewed to investigate the incidence of the lymphocytic infiltration in the histological slides and its possible relevance to a better clinical course. The group that exhibited a definite lymphocytic infiltration (Group A, 11.5%) had a significantly longer preoperative history and postoperative survival (p less than 0.01) than the other two groups that presented slight or no infiltration (Group B, 23%, and Group C, 65%, respectively). In addition, biopsies of 28 recidivous gliomas were reviewed to study the fate of this lymphocytic infiltration in relation to time and therapy, such as irradiation and steroids which are known to depress the immune response. The authors found that severe lymphocytic infiltration is a rare immunobiological reaction which significantly improves the prognosis of a malignant brain tumor and seems not to be influenced by time, local x-ray therapy, or steroids.