Eighty-eight specimens consisting of lymphocytes separated from peripheral blood samples from 76 patients with dengue hemorrhagic fever (DHF) were studied. The results revealed a significant increase in numbers of atypical lymphocytes and B lymphocytes, and a decrease in T lymphocytes by comparison with normal controls. Anti-T lymphocyte antibody was also detected in the serum of all patients with DHF. The greatest increments and decrements of the above were noted on the day of shock of subsidence of fever.