Abstract: Objective　To explore the role of the lymphocyte subsets in the peripheral blood in diagnosis, treatment and prognosis of hemophagocytic lymphohistiocytosis (HLH) in children. Method　A total of 30 children with HLH were enrolled in this study and treated according to the HLH-2004 diagnostic guidelines. 20 children with HLH entered complete remission (CR) and 10 children with HLH died. Thirty age-matched healthy children were selected as normal controls. T cell subsets in the peripheral blood were measured by flow cytometry. Results　Compared with control group, CD3+T and CD8+T cells were significantly increased, CD4+T and CD3-CDl6+CD56+ NK cells were significantly decreased, and CD4+/CD8+ cell ratio was significantly decreased in 20 CR children and 10 died children with HLH in acute phase (P<0.05). CD19+B cells was not statistically different in 20 CR children and 10 died children with HLH in acute phase from control group (P>0.05). In acute phase, the lymphocyte subsets were not statistically different between 20 CR children and 10 died children (P>0.05). In 20 CR children, the proportion of CD3-CD16+CD56+NK in CR phase was statistically different than that in acute phase (P<0.05). Conclusions　Children with HLH have obvious changes in peripheral blood lymphocyte subsets and have cellular immunity disorders. Dynamic detection of the changes may help determine the therapeutic effect and prognosis of HLH.