This study examined the possible mechanism of sublingual immunotherapy (SLIT) in the treatment of allergic asthma. Forty asthma patients allergic to dust mite were enrolled. They received SLIT with dermatophagoides farinae (Der. f) drops for one year. Thirty healthy subjects served as controls. The levels of IL-4 and IFN-gamma of peripheral blood mononuclear cells (PBMCs) were determined in allergic asthma patients before and after the SLIT as well as the healthy subjects. The results showed that the level of IL-4 was substantially increased and that of IFN-gamma remarkably decreased in the patients before the SLIT as compared with those in the healthy subjects (P<0.05). After the SLIT, the level of IL-4 was significantly reduced and that of IFN-gamma elevated in these allergic asthma patients. It was concluded that sublingual immunotherapy is effective for patients with allergic asthma. And it may work by regulating the balance of Th1/Th2 through changing the expression of IL-4 and IFN-gamma in PBMCs.