Chlamydia trachomatis is associated with infections of the mucous membranes of the urogenital system, the upper respiratory tract, and the eye. In industrialized countries, C. trachomatis usually causes sexually transmitted disease. In developing countries, it is the major cause of preventable blindness (trachomatis). Sexually transmitted diseases caused by C. trachomatis include nongonococcal urethritis, cervicitis, salpingitis, epididymitis, proctitis and Lymphogranuloma venereum. Infections are often asymptomatic. Thus, infected individuals may unknowingly transmit chlamydial disease to others. Coinfection with C. trachomatis and Neisseria gonorrhoeae is common, with multiple chlamydial infections of the upper respiratory tract occurring primarily in newborns exposed at parturition through an infected birth canal. Approximately 10-20% of such infants develop pneumonia and 50% develop inclusion conjunctivitis. In adults, eye disease is often transmitted by the hands from genital secretions or from eye secretion of infected babies. Isolation in tissue culture remains the reference method for diagnosis of chlamydial infection, especially when testing individuals under the age of 13 years. The usefulness of serological tests depends on the site of infection, duration of disease, infecting serovars and pre-vious exposure to chlamydial antigens. Because C. trachomatis is ubiquitous, there is a high prevalence of antibodies in sexually active populations. Antibodies may persist even after treatment, making assessment of a single IgG titer difficult. Consequently, serological diag-nosis is seldom used to diagnose active, superficial genital tract infections.