BACKGROUND & OBJECTIVE: Morphine has been proved to inhibit human immune system, and large dose of fentanyl also decrease the activation of natural killer cells. This study was designed to investigate the effects of different doses of fentanyl on T-lymphocyte subpopulations and natural killer cells during esophageal cancer surgery under general anesthesia. METHODS: Forty-five patients with esophageal cancer were randomly divided into 3 groups (I, II,III) with 15 cases in each group respectively. The doses of fentanyl in three groups were 5, 10, and 20 microg/kg, respectively. Central venous blood samples (6 ml) were collected before anesthesia, 24 hours and 48 hours after the operation, respectively. Monoclonal antibodies assay was used to identify T cells and NK cells. RESULTS: The counts of CD3(+) (T%) (Group I 50.30+/-8.42, Group II 48.53+/-9.62, GroupIII 46.58+/-8.56), CD4(+) (T%) (Group I 30.04+/-7.24, Group II 28.67+/-7.52, Group III 26.65+/-6.55),and NK cells (Group I 3.26+/-1.62, Group II 3.01+1.56, GroupIII 3.01+/-1.54) in three groups decreased significantly at 24 hours after surgery (P< 0.01). The decrease at 48 hours after surgery were more significant in groupIII (CD3(+) 48.89+/-9.82, CD4(+) 22.64+/-6.02, NK Cells 3.41+/-1.88) than in group I(CD3(+) 57.32+/-9.13, CD4(+) 35.62+/-5.98, NK cells 5.96+/-1.08) and group II(CD3(+) 55.62+/-10.21, CD4(+) 34.24+/-6.85, NK cells 6.04+/-1.09) (P< 0.05). There was no statistical significance in the counts of CD3(+), CD4(+), and NK cells between the time of 48 hours after the operation and preoperation in group I and group II. CONCLUSION: Fentanyl, as a kind of opiate drug, could contribute to the immunosuppression, and large-dose fentanyl administration would be more effective in suppression of immunity function than small dose fentanyl.