There are four species of Chlamydia: Chlamydia trachomatis, Chlamydia pneumoniae, Chlamydia psittaci, and Chlamydia pecorum. Several significant disease processes in humans are caused by chlamydial infection with C. trachomatis, C. pneumoniae, and C. psittaci. Recognized as a cause of cervicitis, urethritis, and other anogenital infections only in the last three decades, C. trachomatis is a leading cause of sexually transmitted disease in the United States and the leading cause of preventable blindness in developing nations. Several recent advances in the understanding of the biology, natural history, and laboratory diagnosis of chlamydial infections deserve emphasis. These include: a) an increasing awareness of the magnitude of asymptomatic chlamydial infection in both men and women; b) evidence of the organism's role in pelvic inflammatory disease (PID), ectopic pregnancy, tubal infertility, and chronic pelvic pain; c) demonstration of the effectiveness of screening for chlamydial cervical infection in preventing subsequent symptomatic PID; d) the development of more sensitive diagnostic tests using nucleic acid amplification techniques; e) the successful use of these tests on new types of specimens including [lIst void urine and self-collected vaginal swabs; and f) the relative insensitivity and laboratoryto-laboratory variability of chlamydial culture and other non-amplified diagnostic assays.