BACKGROUND: The object of this study was to evaluate apoptosis of peripheral blood mononuclear cells (PBMC) in patients with aseptic meningitis and to determine whether Fas-FasL system is involved in inducing apoptosis. METHODS: Twenty-six patients diagnosed as aseptic meningitis, 10 patients with fever due to upper respiratory tract infection, and 10 healthy children were included in the study. Peripheral mononuclear cells were isolated and after 5h, 24h, 48h incubation, cells were stained with Annexin V and Propidium Iodide and were analyzed with FACScan flowcytometer. The serum levels of soluble Fas were measured by using ELISA. mRNA expression of FasL was evaluated with RT-PCR. RESULTS: The mean rate of early apoptosis of PBMC in the patient group after 24 hr incubation was higher than that of the control groups. The mean rate of total apoptosis in the patient group was higher than that in the normal control group (P<0.01) but was not different from the febrile control group. The mean rates of early and total apoptosis at 48 hours in the patient group were significantly higher than those of the other two control groups (P<0.01). The mean levels of soluble Fas in the subacute stage of the disease were higher than those of in the acute stage of the disease (P<0.05). mRNA of PBMC of the patient group in the acute stage of the disease all expressed FasL. CONCLUSION: The decrease of peripheral lymphocytes in patients with aseptic meningitis may be caused by upregulated apoptosis and Fas-FasL may be involved in inducing apoptosis.