ABSTRACT:Chlamydia trachomatis recently has emerged as the most frequently diagnosed sexually transmitted disease (STD) agent. An estimated three million cases are diagnosed annually in the United States. Because the infection is not reportable, only estimates can be made. The most common STD, it also is often transmitted to infants through infected mothers. Prevention and treatment of the disease in newborns require different procedures from those for gonorrhea, as do diagnostic and treatment methods for infected adults. Chlamydia also appears to pose a threat to pregnancy.