Infection on foreign body: bacterial colonization of ureteric stents. The most frequent cause of the early removal of ureteric endoprostheses (double J) is generally due to bacterial colonization. In order to prevent or to restrict the prosthesis colonization, it is necessary to understand the major steps and the factors influencing the colonization. This is the reason why we aimed to extract the most relevant parameters influencing the bacterial colonization from the observations made in vivo thanks to in vitro analyses. We have studied in vivo the relationship between the bacterial colonization of the endoprostheses, the urinary infections and the antibiotherapy. In vitro, we have defined the conditions promoting the primary adhesion of the most frequently isolated bacteria on endoprostheses. Surface properties of bacteria and materials have been compared to:--the bacterial count of infected double J samples with respect to bacterial species,--the bacterial count of the infected samples with respect to pH and Ca2+, Mg2+ concentration. The results show a great variability of the biomaterial surface properties which could be optimized, the fact that the urinary medium acidification could lower the bacterial adhesion and the ambiguous role of Ca2+ and Mg2+ ions which is discussed in this paper. In the case of in vivo analyses, the conflicting results between leukocyturia and bacteriuria lead to the detection of the bacterial colonization under antibiotic treatment. The characterized urinary infection must warn the risk of pyelonephritis.