Acute and chronic systemic administration of morphine is known to suppress immune function; however, the effect of chronic intrathecal (IT) morphine on immune function in inflammatory-induced pain is still unclear. This study examined the effects on the immune system of IT morphine in rats with formalin-induced pain. Lumbar IT catheters were implanted in rats and saline or 2.5, 5.0 or 10.0 microg/h morphine were administered for 7 days. On the last day, formalin-induced inflammatory pain was induced in rat hind paws and pain intensity was assessed. Rat spleens were then harvested for immune function assay. The IT morphine induced a dose-dependent analgesic effect and lactic acid dehydrogenase release assay showed dose-dependent suppression of natural killer cell activity. Concanavalin-A-induced splenocyte proliferation assay showed IT morphine to suppress T lymphocyte function in a dose-dependent manner. Flow cytometry showed IT morphine significantly to decrease T lymphocyte function and the percentages of T lymphocyte subsets in a dose-dependent manner. Hence, in inflammatory-induced pain IT morphine was found to suppress immune function. Chronic IT morphine should be used cautiously to treat chronic pain in immunocompromised cases.