OBJECTIVE To explore the clinical value of plasma-soluble Fas (Fas) in patients with lung carcinoma. METHODS Plasma-Fas level was measured by ELISA method. T lymphocyte phenotype was analysed by flow cytometry in 60 patients with lung cancer. RESULTS The level of Fas was higher in 40% patients. There was no relationship between level of Fas and pathology. But level of Fas was associated with clinical stage, there was higher level of Fas in patients with advanced neoplastic disease than early stage. The frequencies of activated T cells (CD3+, HLA-DR+) and NK cells (CD16+, CD56+) were also significantly increased in advanced neoplastic patients. However, there was a significant decrease in helper T cells (CD4+, CD8-) in advanced patients. Primary diagnosed SCLC patients with elevated Fas often had good responses to chemotherapy. Plasma Fas level decreased to normal through effective therapy. CONCLUSIONS There is a elevated plasma-Fas level in patients with lung cancer. It is associated with lymphocyte phenotype. Plasma-Fas may be used as a useful parameter in predicting response to therapy or disease activity.