A serological based study was conducted in Sidoardjo, East Jawa, to evaluate the effectiveness of mass larvaciding of Aedes mosquitoes. Seven hundreds and fourty four children (7 y.o.) from endemic areas and another 742 children from non-endemic DHF areas were randomly selected to measure the difference of transmission rate at one month interval, before and after mass larvaciding was excecuted. Abate SG at 1 ppm were used for mass larvaciding in an endemic area only with total coverage 93.4%. Thirthy seven percent of reduction in transmission rate was achieved by mass larvaciding compared with 110% of increase in transmission occuring in the control areas. A level of 2,9% transmission rate in a population with a 100% infection rate will still result in DHF occurance although in a very small number.