The long-term effect of 4 strategies for control of bancroftian filariasis using mass diethylcarbamazine (DEC) chemotherapy was assessed and compared in 4 endemic communities in Tanzania over a period of 4 years.The strategies were the standard 12 d treatment (strategy I), semi-annual single dose treatment (strategy II), monthly low dose treatment (strategy III), and DEC-medicated salt treatment (strategy IV). Treatment was given only during the first year. All strategies resulted in considerable reductions in microfilaraemia, with maximum effects occurring l-2 years after start of treatment. At 2 years, the greatest reductions were seen for strategies III and IV, followed by strategy II and finally strategy I. The overall performance of the 4 strategies evaluated over the 4 years period followed the same sequence. Between the 2 years and 4 years follow-up surveys, a significant increase in microfilarial (mf) burden occurred in all 4 communities, but the mf geometric mean intensities (GMI) remained low. Thus, in individuals who were microfilaraemic before treatment, the rates of microfilaraemia were 66%, 44%, 34% and 43%, and the mf GMIs were 6.8%, 3.3%, 05% and 0.7%, of pre-treatment level, 4 years after start of treatment with strategies I, II, III and IV, respectively. Most individuals who developed microfilaraemia between the 2 years and 4 years follow-up surveys had been microfilaraemic before the start of treatment. Hence, the rate of development of microfilaraemia was much higher (18 times on average in the 4 communities) among those who were microfilaraemic before treatment than among those who were amicrofilaraemic. The long-lasting effect of treatment adds a promising potential to the use of mass DEC chemotherapy for the control of bancroftian filariasis.