We examined the expression of chemokine receptors on the surfaces of T cells and B cells from 27 individuals either with lymphatic filarial disease (lymphedema), with the asymptomatic or subclinical form of filarial infection, or without filarial infection. Individuals with lymphedema exhibited increased percentages of CCR9-expressing T cells and CCR9-expressing B cells and decreased percentages of both CXCR1-and-CXCR3-expressing T cells and CXCR1-and-CXCR3-expressing B cells, compared with asymptomatic or uninfected individuals. A significant correlation was found between the grade of lymphedema and the percentage of CCR9-expressing T cells and CCR9-expressing B cells. The percentages of CCR9-expressing T cells and CCR9-expressing B cells from patients with lymphedema was significantly up-regulated in response to live, infective-stage larvae of Brugia malayi but not to microfilariae of this parasite. Finally, individuals with lymphedema had significantly higher concentrations of interleukin-8, macrophage inflammatory protein (MIP)-1alpha , MIP-1beta , monocyte chemotactic protein 1, thymus-and-activation-regulated chemokine, and interferon-inducible protein 10 in their serum than did uninfected individuals. These results suggest that chemokine receptors (particularly CCR9) are involved in the pathogenesis of lymphatic filarial disease and that trafficking of particular cellular subsets may influence clinical outcome.