Morphine has been reported to suppress human immune response. We aimed to observe the effects of morphine, fentanyl and tramadol on NF-kappa B and IL-2 from both laboratory and clinical perspective. Jurkat cells were incubated with ten times clinically relevant concentrations of morphine, fentanyl and tramadol before being stimulated with PMA. NF-kappa B binding activity and IL-2 levels were measured. In the clinical study, 150 consenting patients were randomized into 3 groups according to the analgesics used in them, namely, group morphine (M), group fentanyl (F) and group tramadol (T). IL-2 was measured preoperatively and 1, 3 and 24 h after operation. Consequently, NF-kappa B activation was suppressed by morphine and fentanyl but not by tramadol. IL-2 was significantly decreased by morphine and fentanyl but not by tramadol in vitro. In the PCA patients, IL-2 was decreased in group M and increased in group F postoperatively. Whereas in group T, IL-2 was unchanged 1 h after operation but was significantly elevated 3 and 24 h after operation. Our results showed that the inhibition of morphine on IL-2 was most probably related to its suppression on NF-kappa B. Fentanyl had different effects on human immune response in vitro and in vivo. Tramadol may have immune enhancing effect.