Antigen presentation and immune activation are essential to the effective control of infectious diseases. In 485 North American adolescents at high risk for genital Chlamydia trachomatis infection, we found 2 human leukocyte antigen variants (DRB1*03-DQB1*04 and DQB1*06) to be associated with recurrent Chlamydia infection (adjusted relative odds [RO], >2.0; P<.01, for both variants). A G-C-C haplotype corresponding to variants at IL10 (encoding interleukin-10 [IL-10]) promoter positions -1082, -819, and -592 was under-represented in individuals with recurrent infection (RO, 0.59; P = .046). These genetic associations were independent of nongenetic factors, including number of sex partners, race, sex, duration of follow-up, and human immunodeficiency virus type 1 seropositivity. Consistent with the observed IL10 association, cervical secretions in female adolescents without the IL10 G-C-C haplotype had elevated IL-10 concentrations after Chlamydia infection, which may reflect involvement of a Chlamydia-specific mechanism for genetically mediated, differential IL-10 expression in the genital tract. © 2005 by the Infectious Diseases Society of America. All rights reserved.