OBJECTIVE: We sought to determine the midtrimester prevalence of Mycoplasma genitalium in women who had subsequent spontaneous preterm birth. STUDY DESIGN: In a prospective study of lower genital tract infections, we identified 127 women who subsequently had spontaneous preterm birth. Vaginal samples were obtained between 21 and 25 weeks' gestation for pH, for bacterial vaginosis Gram stain, and cultures that yielded Chlamydia trachomatis, Neisseria gonorrhoeae, and Trichomonas vaginalis. M genitalium was identified by using validated polymerase chain reaction (PCR) primers, and the results were compared to pregnancy outcomes. RESULTS: Of 124 women with spontaneous preterm births, only five (3.9%) had PCR assays positive for M genitalium. The mean ± SD delivery gestational age was similar for women With a positive PCR (34.6 ± 2.2 weeks) and a negative PCR (34.0 ± 2.7 weeks) (P = .62). None of the women with positive PCR results tested positive for any other sexually transmitted disease, whereas 36 (30%) women with negative PCR results tested positive. CONCLUSIONS: The occurrence of M genitalium in the vagina at midtrimester is infrequent in women with subsequent spontaneous preterm birth.