Objective To investigate the dynamic change of T-lymphocyte subpopulations in the patients with SARS to understand the pathogenesis of SARS,and to find the specific early diagnostic and warning indexes for SARS.Methods T-lymphocytes in the peripheral blood were labelled by specific fluorescein antibodies and examined by flow cytometry in 252 cases of patients with SARS.The relationship between the dynamic change of T-lymphocyte subpopulations and the clinical progression was analyzed.Results The CD 3 +,CD 4 + and CD 8 + T cells,especially CD 4 + T cells decreased at the fist two-week of the disease course and reached to the lowest level at the second week,and began to increase from the third week in all patients.The change pattern of CD 3 +,CD 4 + and CD 8 + T cells in the severe type of SARS was the same as that of the mild type of SARS,but with more serious extent and longer time.The CD 4 + cell amount of the first,second and third weeks in the mild type of SARS was 515?444/?l,421?350/?l and 600?389/?l respectively,and was up to normal level at the fourth week.The CD 4 + cell amount of the first,second,third and fourth weeks in the severe type of SARS was 312?158/?l,255?187/?l,435?343/?l and 539?394/?l respectively,and still lower than normal level at the fourth week.There was a significant difference in the CD 3 +,CD 4 + and CD 8 + cell amount among the different weeks of disease course in both types.NK cell amount also decreased during the course of disease.The CD 4 +/CD 8 + ratio did not change much and was within the normal range.Conclusions The decrease of CD 3 +,CD 4 + and CD 8 + T cells is a sensitive index for early diagnosis of SARS.The continuous decrease of them may indicate the severe condition of the patients with SARS.