T lymphocyte subsets of peripheral blood were studied in preoperative patients with various types of intracranial neoplasms. The subsets were analysed using monoclonal antibodies against lymphocyte membrane markers, and flow cytometry was used to quantitate percent positive cells with the antibodies. Twenty-seven patients were selected for this study, including twelve patients with malignant primary intrinsic tumors histologically consistent with a diagnosis of malignant astrocytoma or glioblastoma multiforme, and fifteen patients with extrinsic tumors diagnosed as meningioma, pituitary adenoma, craniopharygnioma and neurinoma. Twenty-five age and sex-matched individuals without evidence of either local or systemic disorders served as control subjects. The results revealed that the OKT4/8 cell ratio was 1.4 +/- 0.4 in the malignant group, 1.8 +/- 0.4 in the benign group and 2.1 +/- 0.8 in the control group. The ratio was significantly lower in the malignant group than in the control group (p less than 0.05). Leu-11+ cells were found to be 9.7 +/- 4.7 in the malignant group, 9.0 +/- 3.4 in the benign and 7.8 +/- 2.7 in the control group. These results showed that Leu-11+ cells in the patients with malignant tumors were significantly increased in comparison with the control group (p less than 0.05). The alteration of the lymphocyte subsets is considered to be an effect of neurohormones in balance through the transmission function of the brain-endocrine axis to lymphocytes in immunomodulation.