Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the United States with 10 million cases annually. Studies have been carried out to further define the clinical spectrum of chlamydia infection, to develop rapid diagnostic assays, and to examine the pathogenesis of chlamydial infections in experimental animal models. In a study of 542 pregnant women, chlamydia was present in 16% and was significantly associated with the clinical findings of cervicitis, preterm delivery, low birth weight infants, and conjunctivitis. In a subsequent study of 505 pregnant women presenting for induced abortions, C. trachomatis, present in 17% of the patients, was also significantly associated with the development of postabortal endometritis. Newly developed diagnostic assays, such as immunofluorescent staining with monoclonal antibodies, and in situ DNA hybridization were shown to have a relatively high sensitivity and specificity for the detection of chlamydia. Development of a primate model of rectal C. trachomatis lymphogranuloma venereum (LGV) infection has aided in the study of the mucosal immune response to infection. Following rectal infection, there was reversal in the systemic T-cell lymphocyte populations and T-cell antigen specific responses to LGV were demonstrated in the peripheral blood and mesenteric lymph nodes, but not in the lamina propria. This failure to respond to C. trachomatis antigen within the mucosa correlated with persistence of C. trachomatis in tissue histiocytes as determined by immunofluorescence and in situ DNA hybridization. In summary, the above studies demonstrate the diverse clinical spectrum of C. trachomatis infection, and provide methods for the rapid screening of chlamydia and for studying the mucosal immune response to C. trachomatis.