The biopsy specimens of 342 patients with malignant glioma were evaluated to determine the extent and prognostic significance of round cell infiltration within these representative tissues: (a) tumor, (b) peritumoral, (c) hypervascular, (d) necrotic, and (e) normal tissue. Thirty-six percent of all biopsy specimens showed at least one tissue area with round cell infiltration. Patients in age groups 0 to 25 years and 71 years and older tended to show less round cell infiltration than did patients 41 to 55 years old. The presence of round cell infiltration in tissue was associated with a poor prognosis. Patients showing any infiltration had mean survival times of 8.4 months as opposed to 11.9 months for those showing no infiltration. The relationship between round cell infiltration and poor prognosis is true, irrespective of postoperative therapy, sex, and age.