Improving knowledge about breast cancer etiology is crucial in order to propose preventionstrategies for this pathology. Gut microbiota is involved in numerous physiopathological situationsincluding cancers. Although its potential involvement in breast cancer through the alteration of theenterohepatic circulation of estrogens and/or the metabolism of phytoestrogens has beendiscussed for some time, it remains to be demonstrated. The present study seeks to strengthen thishypothesis by identifying possible links between the fecal microbiota composition and clinicalcharacteristics in breast cancer patients. Bacterial DNA was extracted from the feces of 31 patientswith early-stage breast cancer and amplified by real-time polymerase chain reaction (qPCR),targeting 16S rRNA sequences specific to bacterial groups, and then analyzed in relation to clinicalcharacteristics. The absolute numbers of total bacteria and of three bacterial groups (Firmicutes,Faecalibacterium prausnitzii, and Blautia) differed significantly according to the patient’s body massindex. The percentage and the absolute numbers of certain bacterial groups, namely C. coccoides, F.prausnitzii, and Blautia, differed significantly according to the clinical stages and the histoprognosticgrades. Our study highlighted that intestinal microbiota composition in these patients differsaccording to clinical characteristics and BMI. Further studies are required to clarify the link betweenbreast cancer and intestinal microbiota.