To clarify the relationship between urinary tract infections and stone formation, infected renal calculi removed surgically from 19 patients were investigated. First, the stones were studied using a scanning electron microscope and bacteriological method. Most of the stone cores consisted of calcium phosphate and organic materials containing bacteria, fibrin, erythrocytes, leukocytes and so on. Second, in experimental ascending pyelonephritis in rats which received the intravesical instillation of Proteus mirabilis, the incidence of renal stone formation was increased with the grade of the pyelonephritis and necrotic papillae played an important role as stone nuclei. Third, urinary materials, which may initiate and accelerate stone formation, were investigated using the urine of stone formers associated with renal infections, nucleopore filters and stitches, from the standpoint of crystal aggregation and adhesion effects. The bacteria tended to aggregate crystalline and organic matters in the urine and to adhere them to the stitches before the crystals. The results obtained suggest that the bacteria and organic matters in the urine of stone formers participate actively in stone genesis and growth as an adhesive agent.