BACKGROUND Endoscopic placement of a iodine-125 radioactive stent is useful to treat obstructive jaundice with unresectable periampullary tumors. This study aimed to retrospectively evaluate the obstructive component of biliary radioactive stents and discuss the different obstructive mechanism with common plastic stents. METHODS Twenty consecutive patients with malignant obstruction underwent insertion of stents into the common bile duct, including 10 radioactive stents and 10 polyethylene stents. The radioactive stents were withdrawn after ∼3 months or earlier if clinical signs suggested stent clogging. Polyethylene stents were withdrawn after physical signs suggested stent clogging. Bacteriologic analyses included identification of aerobic and anaerobic bacteria. Stent surfaces were observed by scanning electron microscopy. Stent deposition was identified by Fourier-transformed infrared spectroscopy and pyrolysis derivatization/gas chromatography/mass spectrometry. RESULTS Radioactive stent group and polyethylene stent group stents were placed for 86 days (interquartile range 62, 114) and 146 days (interquartile range 105, 181) respectively. The placement duration of the two types of stents was statistically significant. A variety of microorganisms were cultured from the stent deposits. Scanning electron microscope images showed a thicker necrotic layer on the external surface of polyethylene stent than the radioactive stent group. The proportions of obstructive components in each stent were different, but none of them were statistically significant. Necrotic tumor tissue was found in the radioactive stent group. CONCLUSIONS Similar clogging events occurred in both radioactive stents and polyethylene stents. The median duration time of the radioactive stent was shorter, probably because of the smaller inner diameter, and the radioactive seeds exerted no beneficial effect in inhibiting microorganisms.