Abstract: Objective　To investigate expression of 4-1BB and lymphocyte subsets in peripheral blood of children with acute infectious lymphocytosis. Methods Flow cytometry (FCM) was applied to detect the expression of 4-1BB and lymphocyte subsets in peripheral blood of 15 cases of acute infectious lymphocytosis and 20 cases of acute upper respiratory infection, and 20 healthy children. Results The expression of 4-1BB and CD3+, CD4+ and CD8+ lymphocytes were higher in acute infectious lymphocytosis group than those in acute upper respiratory infection group and healthy control group (P<0.05). There was no significant difference of CD19+CD23+ lymphocytes among three groups (P>0.05). A positive correlation was found between 4-1BB expression and CD3+ lymphocytes expression in acute infectious lymphocytosis group (r=0.73, P<0.05). Conclusions The abnormal expression of 4-1BB may play a pathological role in the development of acute infectious lymphocytosis. T cells in children with acute infectious lymphocytosis may not function to activate B cells.