BACKGROUND. Single dose diethylcarbamazine (DEC) as used in control programmes is effectively microfilaricidal for periods of up to a year or more but has incomplete ability to kill Wuchereria bancrofti adult parasites. These regimens can be effective in breaking transmission by suppression of circulating microfilariae available to mosquito vectors. Whether prolonged or aggressive therapy with DEC has a significant effect on adult worms, which may live up to 12 years or more, and is important in the context of the treatment of individual patients, is still incompletely understood. METHODS. In order to investigate the adulticidal effect of aggressive therapy, DEC was given at 6 mg/kg/day for 12-day courses at 0, 6, 12, and 18 months and Og4C3 antigenaemia followed over two years in 38 CAg + Brazilians in a W. bancrofti endemic area. RESULTS At two year follow-up, the median level of antigenaemia was 21% of the pre-treatment value. 92% of individuals had antigen levels < 50% of pretreatment values, but only 26% had completely cleared antigenaemia. The clearance rate at 24 months was only 12% (3/26) in the asymptomatic CAg + patients but 58% (7/12) in those with clinical manifestations of filariasis. The latter individuals cleared significantly more antigen (median of 0% pretreatment antigenaemia vs. 26%; P = 0.02) than asymptomatic but infected individuals. CONCLUSION. Aggressive repeated therapy with DEC alone is ineffective in consistently eradicating adult W. bancrofti, especially in infected but asymptomatic individuals. Prolonged courses of combination therapy with other antifilarial drugs should be investigated for treatment of individual patients with the means to pursue aggressive personal medical care.