OBJECTIVE To test in vitro stents made from a new biomaterial that is less liable to encourage adhesion of bacteria that may lead to blockage of the stent. DESIGN Laboratory experiment. SETTING University hospital, Italy. MATERIAL 15 polyethylene biliary endoprostheses that had been removed endoscopically a mean of 151 days (range 55416) after insertion. PUPA, a biomaterial that contains polyamidoamine cross-linked to polyurethane chains. This can bind large quantities of heparin and HyalSx (hyaluronic acid at a different stage of sulphation) in a stable fashion. MAIN OUTCOME MEASURES Incidence of blockage and growth of pathogens in the polyethylene biliary prostheses. Adhesion of pathogens to PUPA in vitro on electron microscopy. RESULTS 12 of 15 polyethylene prostheses were blocked by brown concretions composed of calcium bilirubinate, palmitate, and little cholesterol. All concretions grew more than one pathogen, and the growth always included Escherichia coli. Of the 5 PUPA stents tested, only 1 had bacteria sticking to their surfaces. CONCLUSION These results confirm previous studies that showed that HyalSx appreciably inhibited the adhesion of bacteria and is therefore a suitable material for the manufacture of biliary stents.