Rationale To evaluate the role of peripheral blood mononuclear cells (MO) and polymorphonuclear leukocytes (PMN) in the prolonged inflammation, we examined ROS generation of those cells in AD. Methods 17 AD patients and 10 healthy controls were collected and isolated MO and PMN were stimulated with distinct reagents, phorbol ester (PMA), adenosine triphosphate (ATP), chemotactic peptide (f-MLP), respectively. ROS levels were measured using luminol-dependent chemiluminescence assay. Results In atopic dermatitis, chemiluminescence response of unstimulated MO was higher than that of normal controls. Although all the stimuli increased chemiluminescence activity of MO, only the response by PMA was significant. Additionally, ROS generation tendency to PMA was different between AD and normal controls. In normal controls, a steadily increased trend by PMA, but in AD, the response was initial peak increase and then gradually decreased. In contrast, the response of unstimulated PMN from AD was similar to healthy controls and was rather steadily decreased after stimulation. Conclusions These results indicate that MO from AD, but not PMN, is primed and ready to be activated in vivo, and no functional defect exist in PMN from AD. We conclude that enhanced respiratory burst activity of MO is implicated in the prolonged inflammation of AD.