The objectives of this project are to define the epidemiology, risk factors, transmission kinetics and pathogenesis of Chlamydia trachomatis infections in different population settings and different disease states using new molecular amplification assays. To address this objective, we implemented non-invasive screening with molecular assays for C. trachomatis and Neisseria gonorrhoeae, and documented high rates of infection in sexually active adolescents in diverse cultural settings in six countries. Prevalence rates of chlamydia ranged from 3.8% in rural villages in Zimbabwe and Uganda, 12.5% in St. Petersburg, Russia, 11.5% in Lima, Peru, 16.7% in Fuzhou, China, and over 20% in Baltimore, USA. Women generally had higher prevalence rates of C. trachomatis in 16 to 20-year-olds compared to men with prevalence declining in both genders in the older age groups. In the US, we screened 25,000 female and 5,000 male military recruits from 50 states and found a prevalence of 9.2% in women and 5.3% in men. Chlamydia infection was associated with multiple sex partners, lack of condom use, and young age. 80% of those infected were asymptomatic. In a household survey in Baltimore, of 579 adults aged 18 to 35, 7.9% of the population had either untreated gonococcal or chlamydial infection with an estimated prevalence substantially higher among African American females (15%). In a related study of 434 patients aged 18 to 31 years attending the Johns Hopkins Emergency Department, 13.6% of patients were infected with chlamydia or gonococcal infection. Only a quarter were initially suspected and treated for their STD in the Emergency Department. These studies document the widespread infection of C. trachomatis infections domestically and worldwide. Of increasing importance to this laboratory is the investigation of the role of Chlamydia pneumoniae in the development of atherosclerosis utilizing seroepidemiologic, pathologic, and animal models. We have found evidence of C. pneumoniae infection in 50% of coronary and carotid atheromas by immunocytochemistry and/or PCR. In addition, we documented that C. pneumoniae IgG titer correlated with severity of allograft arteriosclerosis after cardiac transplantation. Circulating C. pneumoniae DNA was detectable by PCR in up to 30% of cardiac transplant recipients but did not correlate with severity of allograft vasculopathy. In a preliminary animal model study, we were able to accelerate atherosclerotic lesions of the aorta with C. pneumoniae infection in apoE-deficient mice at a greater rate than non-infected control mice. Co-infection with cytomegalovirus also appeared to further potentiate the atherosclerotic process.