Our objective was to comprehensively test the composition, morphology, and structure of enlarged prostatic utricle (EPU) stones, which, as a kind of biolite in abnormal anatomical structures of the genitourinary system, have not yet been reported. Thirty-one EPU stones coming from 8 patients, who had been treated in our center from 1985 to 2009, were taken out by transurethral fenestration of EPU ... [Show full abstract] and were analyzed by scanning electron microscopy, x-ray diffraction (XRD) analysis, and Fourier transformation infrared spectral (FTIS) analysis. Under scanning electron microscope, all these EPU calculi were seen to be constituted of many intensive minicrystals and amorphous matrix. By XRD and FTIS analysis, we determined that the 31 EPU stones in our research were all hydroxyapatite crystal. We consider that EPU calculi should belong to the category of prostatic pseudocalculi and that the formation of EPU calculi is not caused by the abnormal change of urine composition, but should be ascribed to continuously concentrated EPU liquid by absorption of capsule walls and calculous matrix mainly coming from deciduous epithelial cells of EPUs. The role of the amorphous matrix is to link microcrystals, which promotes the growth of EPU stones.