In non-obese patients ketamine decreases inflammatory responses and prevents overexpression of immune re- sponses. Its effect in obese patients is unknown. This prospective, blinded, randomized controlled trial was designed to determine the effect of ketamine on cytokines and immune cell responses after short-duration surgery in obese patients. Thirty-six patients received either ketamine 0.15 mg/kg IV prior to induction of general anesthesia, or an equal volume of normal saline. Cytokine concentrations and immune cell responses were determined pre-operatively and at 4, 24, and 48 h after operation. Interleukin (IL)-6 production was significantly greater in the control group (126.0 ± 18.8 ng/ml, mean ± SEM, n = 19) than in the ketamine group (57.9 ± 8.4 ng/ml) at 4 h. At other time periods IL-6 and tumor necrosis factor � increased and IL-2, lymphocyte proliferation, and natural killer cell cytotoxity decreased compared to pre-operative values in the control group but not in the ketamine group. We conclude that effects of ketamine on inflammatory and immune re- sponses after short-duration surgery in obese patients are similar to those previously reported in non-obese patients.