['To investigate whether the presence of cervical microorganisms, as detected on catheters used for ET, alters the outcome of IVF-ET.', 'Prospective analysis.', 'The assisted reproduction unit of a hospital in Clamart, France.', 'Two hundred seventy-nine controlled ovarian hyperstimulation (COH) cycles performed for IVF-ET. Inclusion criteria were a patient age of < or =38 years, a morphologically normal uterus, and > or =2 good-quality embryos transferred.', 'The tips of catheters used for ruling out possible cervical obstruction before ET were subjected to quantitative (> or =10 colonies = positive culture group; <10 colonies = negative culture group) and qualitative microbial assessment.', 'Pregnancy and implantation rates.', 'In 143 (51%) of 279 ETs, cultures were positive, predominantly for Escherichia coli (64%) and Streptococcus species (8%). Although data on patients, COH, and embryology were similar in both culture groups, clinical and ongoing pregnancy rates as well as implantation rates were significantly lower in the positive culture group than in the negative culture group (24% versus 37%; 17% versus 28%; and 9% versus 16%, respectively).'] Does microbial flora of the cervix assessed at the time of embryo transfer adversely affect in vitro fertilization outcome?