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Longitudinal studies are being conducted in Leogane, Haiti to investigate the relationship between acquisition of filarial infection and development of antifilarial immunity as well as the impact of maternal infection on this relationship. Children (0-24 months of age) residing in Leogane were enrolled and were examined periodically to monitor parasitologic status and to collect serum for antigen and antifilarial antibody determinations. To examine the development of filarial antigenemia and antifilarial antibody responses in this cohort, serum samples were selected from a cross section of the population at two (n = 82) and four years of age (n = 76). Antigen prevalence increased from 6% among two-year-olds to more than 30% among four-year-olds, but in only one four-year-old child were microfilaria detected in a 20-microl smear. The proportion of antigen-positive children born to antigen-positive mothers was higher than the proportion of antigen-positive children born to antigen-negative mothers (9.8% versus 0% for two-year-olds; P = 0.15; and 39.6% versus 22.7% for four-year-olds; P = 0.18). Antifilarial IgG4 levels were significantly higher among antigen-positive children at both two and four years of age (P < 0.001). In analyses of paired samples, antifilarial IgG4 responses increased significantly more among children who acquired infection by four years of age than among children who remained antigen negative, whereas antifilarial IgG1 and IgG2 responses changed equally for antigen-positive and -negative children. Antifilarial antibody levels were not influenced by maternal infection status, but were significantly influenced by age, antigen status, and the neighborhood within the community. These results provide evidence that children acquire infection early in life and suggest that antifilarial antibody responses may peak in early childhood.