Objective: To investigate whether the presence of cervical microorganisms, as detected on catheters used for ET, alters the outcome of IVF-ET. Design: Prospective analysis. Setting: The assisted reproduction unit of a hospital in Clamart, France. Patient(s): Two hundred seventy-nine controlled ovarian hyperstimulation (COH) cycles performed for IVF-ET. Inclusion criteria were a patient age of less than or equal to 38 years, a morphologically normal uterus, and greater than or equal to 2 good-quality embryos transferred. Intervention(s): The tips of catheters used for ruling out possible cervical obstruction before ET were subjected to quantitative (greater than or equal to 10 colonies = positive culture group; <10 colonies = negative culture group) and qualitative microbial assessment. Main Outcome Measure(s): Pregnancy and implantation rates. Result(s): 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). Conclusion(s): The presence of microbial flora of the cervix on ET catheters is associated with poor IVF-ET outcome. (Fertil Steril(R) 1998;70:866-70. (C) 1998 by American Society for Reproductive Medicine.).