The earliest potential exposure of the human host to filarial antigens is in the uterine environment. The impact of this exposure on host susceptibility and development of antifilarial immune responses following challenge with infective larvae is unclear. Although a number of studies suggest that children born to infected mothers may have a greater risk of acquiring infection than children of uninfected mothers, it has been difficult to relate these epidemiologic findings to specific alterations in antifilarial immune responsiveness. Immunologic studies of cord blood mononuclear cells (CBMC) have demonstrated that CBMC are capable of responding to crude filarial antigens with the production of a range of cytokines and parasite specific antibodies that are qualitatively similar to those produced by adults. Although these data demonstrate that sensitization to filarial antigens does take place in utero, additional work is needed to critically examine CBMC responses to defined parasite antigens and to conduct quantitative comparisons of B and T cell responses of children born to infected and uninfected mothers. In addition, it is important to consider the possibility that susceptibility to filarial infection may not depend strictly on the expression of filaria-specific immune responses per se, but may be a function of the cytokine environment in which the parasite develops. If so, in utero exposure to filarial antigens may alter this milieu.