Our objective was to detect the expression of CD226 on natural killer (NK) cells and T cells, and to measure the amount of soluble CD226 in the plasma of HIV-infected individuals, in order to evaluate the function of CD226 in HIV infection. Thirty-four untreated HIV-1-infected patients and 26 normal controls were enrolled and three-color flow cytometry was used to detect the expression of CD226 on T lymphocytes and NK cells in whole blood samples taken from the patients and normal controls, and in HIV-1SF33-infected peripheral blood mononuclear cells (PBMCs). An enzymelinked immunosorbent assay (ELISA) was used to detect the level of soluble CD226 in the plasma of HIV-infected patients and normal controls and in the supernatant of HIV-1SF33-infected cells. The level of CD226 expression on CD3+, CD4+, and CD8+ T cells and on CD3- CD16+ NK cells of HIV-infected patients was significantly higher than that of normal controls (p < 0.01). The level of soluble CD226 in the plasma of HIV-infected patients was also significantly higher than that of normal controls (p < 0.01). After stimulation with HIV-1SF33, the level of CD226 expression on CD3+ T cells and CD3- CD16+ NK cells of cultured PBMCs reached peak values at 48 h, which was earlier than in uninfected control cells (72 h). The level of soluble CD226 in the supernatant of HIV- 1SF33-infected cell culture was higher than that of uninfected cells, and the level of soluble CD226 in the supernatant of HIV-1SF33-infected cells reached the peak value at 72 h, which was earlier than in uninfected control cells (96 h) but later than the time of peak CD226 expression on CD3+ T lymphocytes (48 h). We conclude that CD226 may be involved in the immune response to HIV infection and that further experiments are needed to find the function of CD226 in the pathogenesis of HIV infection.