In recent years, the use of immunotherapy for malignant tumors has been proposed. To explore the significance of immunotherapy for lung cancer, we examined two systems : the HLA class I and cancer-reactive CTL system, and the Fas-FasL system.HLA class I (HLA-A, -B, and -C) antigens were determined in 61 patients with lung cancer and in 30 healthy controls. The HLA class I phenotype was investigated by serological techniques. HLA-A2 alleles were investigated by polymerase chain reaction sequence-specific primer analysis. We analyzed lymphocytes isolated from 61 patients with two-color surface labeling and flow cytometry. Furthermore, we analyzed sFas and sFasL expression by enzyme-linked immunosorbent assay (ELISA). We also examined lung cancer cell line-induced apoptosis of CD8(+) lymphocytes using confocal laser scanning microscopy.The HLA-A2 allele was observed in 27 of 61 patients with lung cancer. There were no differences in HLA-A2 allele classifications between lung cancer patients and controls. Thirty-six of the 61 lung cancer patients (57%) had elevated levels of sFas, and 16 of the 61 patients (26.2%) had elevated levels of sFasL. The sFas level of lung cancers with HLA-A2 was significantly higher than that of cancers without HLA-A2 ( P<0.01). This tendency was observed in every lung cancer tissue type, and the sFas levels of lung cancers with HLA-A2 associated significantly with the CTL levels of lung cancers with HLA-A2. Furthermore, compared to lung cancers without HLA-A2, CD8(+) T-cell levels were elevated in lung cancers with HLA-A2. In contrast, sFas levels of non-small cell lung cancers without HLA-A2 were higher than those in lung cancers with HLA-A2. In an in vitro experiment using lung cancer cell lines, we observed apoptosis of CD8(+) lymphocytes induced by lung cancer cells. Lung cancer-reactive CTLs are mobilized easily by restriction of HLA-A2, but this restriction is not always specific. In addition, FasL derived from lung cancer cells can induce apoptosis of CD8(+) T-cells, and the frequency of this phenomenon is increased in small cell lung cancers without HLA-A2.Our findings suggest that the effect of immunotherapy may be insufficient for non-small cell lung cancer without HLA-A2.