OBJECTIVE: To define the impact of helminthic infestations and their treatment on viral load and T cell subsets in chronic HIV-1-infected patients. METHODS: Two hundred twenty chronic HIV-1-infected Ethiopian patients with (N = 87) and without (N = 133) helminthic infestations were included. To determine the impact of deworming on viral load and T cell subsets, a subset of these patients with (n = 23) and without (n = 20) helminthic infestations were followed longitudinally. Helminth egg loads, plasma HIV RNA levels, and peripheral blood CD4(+) and CD8(+) T cells were determined at baseline and at 12 weeks after antihelminthic treatment. RESULTS: At baseline, plasma viral load was significantly higher in individuals with (n = 220) than without (n = 87) a helminthic infestation (5.01 log10 vs. 3.41 log10, p < 0.001). Twelve weeks after antihelminthic treatment, plasma HIV RNA levels were reduced in the successfully treated group (p < 0.001). Twelve weeks after antihelminthic treatment, helminth infestations and their treatment had no significant effect on CD4(+) T cell counts. However, helminth-infested individuals had a higher level of CD8(+) T cells at baseline (p < 0.001), which was significantly reduced (p < 0.01) at 12 weeks after antihelminthic treatment. CONCLUSIONS: Helminths were found to be associated with an increased HIV RNA level. Successful treatment of intestinal helminths reduced plasma HIV RNA levels in chronic HIV-1 subtype C infection. Considering the high endemicity of helminths in tropical settings, the management of chronically HIV-infected individuals must include deworming.