The microbiology, laboratory diagnosis, clinical features, and treatment of infectious diseases caused by Chlamydia trachomatis and Chlamydia psittaci are reviewed. Chlamydial genital infection is the most common sexually transmitted disease in the United States. C. trachomatis plays an important role in nongonococcal urethritis, postgonococcal urethritis, cervicitis, pelvic inflammatory disease, and possibly proctitis. Proper treatment of genital infections caused by C. trachomatis is important to prevent severe sequelae such as epididymitis and pelvic inflammatory disease, both of which may lead to sterility. Infected pregnant women should be treated to prevent transmission of chlamydial infections to their infants. Chlamydial conjunctivitis is the most common eye infection in the first month of life. C. trachomatis also causes pneumonia in infants. Lymphogranuloma venereum and trachoma are important diseases in developing countries. C. psittaci, transmitted by birds, causes psittacosis in humans; this disease is rare in the United States. Diagnosis is a problem because chlamydial genital infection can be asymptomatic and because the organism is difficult to isolate. Culture remains the diagnostic method of choice, but recently marketed direct-detection assays provide qualitative results within hours after specimen collection. Tetracyclines and erythromycin are the drugs of choice for treating chlamydial infections in adults. An antimicrobial with activity against both C. trachomatis and Neisseria gonorrhoeae is preferred because both organisms are often present concurrently in patients with sexually transmitted disease. Chlamydial infections in pediatric patients often respond to systemic erythromycin therapy; tetracyclines are equally effective but are contraindicated for children less than nine years of age. Adequate diagnosis and treatment of sexually transmitted chlamydial infections in patients and their contacts is important to limit the spread of disease.