Chlamydia is a common infection among youth and can be given from one person to another during sex. Many people who have chlamydia have no signs of infection at all, but can pass the infection to anyone they have sex with. If not treated, chlamydia can lead to serious health problems. This study will look at how well medicines given for chlamydia infection work. The study requires 306 evaluable subjects, chlamydia-positive, males and non-pregnant females, ages 12-21, living in long-term, gender-segregated youth correctional facilities. Participants will be assigned to receive either doxycycline (2 times per day, by mouth, for 7 days) or azithromycin (1 single dose by mouth). Study procedures will include collection of at least 3 urine samples to test for chlamydia. Study visits will occur during initial enrollment in the study, day 28 after starting treatment, and day 67. Participants will be involved in study related procedures for up to 67 days.