This study was aimed to explore the level of NK cell activity in the patients with secondary hemophagocytic syndrome (HPS) and its significance for early diagnosis of this disease. 16 suspected HPS patients and 25 healthy subjects were enrolled in this study. The activity of NK cells in peripheral blood was detected by a released LDH assay. The activity level of NK cells in peripheral blood from patients who were finally diagnosed as HPS was compared with healthy subjects. The results showed that 8 out of 16 suspected HPS patients were finally diagnosed as secondary HPS. The activity of NK cells in peripheral blood of these 8 patients was obviously lower than that in healthy subjects with statistical significance (p<0.001), and showed abnormal in the early stage of this disease. It is concluded that the detection of NK cell activity may play an important role in earlier diagnosis of secondary hemophagocytic syndrome.