Objective: The objective of the study was to estimate whether midpregnancy genitourinary tract infection with Chlamydia trachomatis is associated with an increased risk of subsequent preterm delivery. Study design: Infection with C. trachomatis was determined using a ligase chain reaction assay (performed in batch after delivery) of voided urine samples collected at the randomization visit (160/7 to 236/7 weeks' gestation) and the follow-up visit (240/7 to 296/7 weeks) among 2470 gravide women with bacterial vaginosis or Trichomonas vaginalis infection enrolled in 2 multicenter randomized antibiotic treatment trials (metronidazole versus. placebo). Results: The overall prevalence of genitourinary tract C. trachomatis infection at both visits was 10%. Preterm delivery less than 37 weeks' or less than 35 weeks' gestational age was not associated with the presence or absence of C. trachomatis infection at either the randomization (less than 37 weeks: 14% versus 13%, P=.58; less than 35 weeks: 6.4% versus 5.5%, P=.55) or the follow-up visit (less than 37 weeks: 13% versus 11%, P=.33; less than 35 weeks: 4.4% versus 3.7, P=.62). Treatment with an antibiotic effective against chlamydia infection was not associated with a statistically significant difference in preterm delivery. Conclusion: In this secondary analysis, midtrimester chlamydia infection was not associated with an increased risk of preterm birth. Treatment of chlamydia was not associated with a decreased frequency of preterm birth. © 2006 Mosby, Inc. All rights reserved.