Mice (C57BL/6), treated with progesterone and infected intravaginally with the mouse pneumonitis strain of Chlamydia trachomatis (MoPn), acquired genital tract disease that ascended from the endocervix to the uterine horns, oviducts, and ovaries in a temporal fashion before the occurrence of spontaneous microbiological resolution by about 28 days after infection. Surprisingly, dissemination of MoPn in small numbers to draining lymph nodes, the peritoneal cavity, spleen, liver, kidneys, and lungs occurred in normal mice during the early stages of disease (7 to 14 days) in a portion of infected animals but resolved from these tissues, by microbiological criteria, prior to resolution of genital tract involvement. In contrast, gamma interferon knockout (IFN-gamma KO) mice exhibited dissemination of infection to a greater extent and for longer periods in a variety of tissues, and a portion of infected IFN-gamma KO mice failed to microbiologically resolve their genital tract disease. By comparison, C57BL/6 SCID mice uniformly failed to resolve their genital tract disease and exhibited high levels of dissemination to all tissues tested for extended (50-day) periods of times. Interestingly, although IFN-gamma KO mice failed to completely clear organisms from their genital tracts, they exhibited an attenuated infection indistinguishable from that of heterozygous littermates when challenged 112 days after primary infection. These data support a role for IFN-gamma in containing dissemination of MoPn from the genital tract to extragenital sites and in the microbiological resolution of infection. Data also indicate that IFN-gamma is not required for modulating reinfections, which normally follow a shorter and less dramatic course.