Objective To investigate the immunologic etiology of pregnancy induced hypertention (PIH)by studying the changes of T H1 and T H2 cell ratio in the peripheral blood from patients with severe PIH. Methods We examined the cell percentage of T H1 cell(secreting cytokine IFN ?) and T H2 cell(secreting cytokine IL 4),which were from CD4 positive, by flow cytometry tri stained with PerCP CD4,FITC IFN ? and PE IL 4 monoclonal antibody. The blood samples were from severe PIH patients and normal third trimester patients. Results The percentage of T H1 cell 38.01?9 04% from the peripheral blood of severe PIH partuients was significantly higher than that of the normal partuients of the third trimester 30.26?8.65%, while the percentage of T H2 cell 2.25? 0.61% was much lower P