In 2002, DHF viro-serological surveillance was started in 19 Southern provinces of Vietnam. 247 DEN virus strains (consist of 27 DEN-1, 99 DEN-2, 9 DEN-3 and 112 DEN-4) were isolated from 3318 blood samples. The predominant serotypes were DEN-4 (45.3%) and DEN-2 (40.1%). DEN-2 virus was more positively correlated with severe cases in comparison with other serotypes. However, high rates of virus isolation were obtained from DHF grade I-II patients (49%), and then from DF, viral infection and fever of unknown origin cases (40.1%). These should be main target subjects for the active surveillance of DHF. In addition, MAC-ELISA was done on 7498 DHF serum samples taken in Southern Vietnam with the average positive rate of 44.37%