Although morphine-induced modulation of immune cells has been well studied, modulation of immune cells by fentanyl and remifentanil, the latter of which has been used in recent anesthesia procedures, has not been well-studied. Our aim was to identify the effects of fentanyl and remifentanil on phagocytosis and respiratory burst in leucocytes in in vivo and in vitro studies. In the in vivo study, twelve patients were assigned to receive fentanyl-based anesthesia (fentanyl group, n=6) or remifentanil-based anesthesia (remifentanil group, n=6). Blood samples were obtained from before anesthesia to 30 min after anesthesia in each group. In the in vitro study, blood samples were obtained from three healthy volunteers and incubated with various blood concentrations of fentanyl or remifentanil (from 0.3 ng/mL to 9 ng/mL). Phagocytic activity (percentage of phagocyting cells) and respiratory burst activity (percentage of cells producing oxygen radicals) were analyzed. In the in vivo study, phagocytosis was suppressed only before incision in the fentanyl group whereas suppression of phagocytosis was continued until emergence from anesthesia in the remifentanil group. In the in vitro study, incubation with fentanyl or remifentanil tended to enhance phagocytic function of monocytes and had no dose-dependent effect over various concentrations of fentanyl or remifentanil, respectively. Remifentanil-based anesthesia suppressed the phagocytic function of monocytes during its administration.