Clinical and blood surveys were made in four villages of Tutuila, American Samoa, in 1962 before and after treatment with diethylcarbamazine (DEC) in order to determine the current prevalence of filariasis, investigate continued transmission, assess the need for control, and evaluate dosages and schedules for drug administration. The microfilaria (mf) rate was 20.4%, the elephantiasis rate 3.4%, and no spontaneous decline of clinical or hematologic filariasis had apparently occurred during the previous 20 years. Therefore, mass treatment with DEC was recommended to a total dose of 72 mg per kg of body weight. This dose reduced the mf rate to 3.9% by the end of the first year, but, regardless of the schedule adopted for its administration, appeared insufficient for adequate control. However, because reductions of mf rate in two villages receiving identical regimens of DEC appeared proportional to their pretreatment level of mf infection, it was suggested that further treatment with DEC be administered to determine its subsequent effect upon mf rates and transmission.