We assessed the effect of anti-filarial treatment (diethylcarbamazine, DEC) on HIV load, CD4%, and CD4/CD8 ratio in HIV-positive individuals with and without infection with the filarial parasite Wuchereria bancrofti in a randomized, double-blind, placebo-controlled cross-over trial. The study was conducted in Tanga Region, Tanzania, in 2002 and involved 27 adults. A significant decrease in HIV load (54%) and an insignificant increase in CD4% were observed in the HIV-positive individuals with filarial co-infection at 12 weeks after treatment. HIV load and CD4% both increased, although not statistically significantly, in the HIV-positive individuals without filarial infection. The findings suggest that DEC affected HIV load through its effect on the filarial infection rather than through a direct (pharmacodynamic) effect on HIV. Global efforts to control lymphatic filariasis by annual mass treatment with DEC may have a beneficial effect on the HIV/AIDS epidemic in areas where HIV and lymphatic filariasis co-exist.