To determine the factors influencing stone fragmentation and to suggest when to change treatment modality for ureteral calculi refractory to repetitive in situ extracorporeal shockwave lithotripsy (SWL), we analyzed 369 patients treated primarily by the second-generation lithotripter, Siemens Lithostar, from March 1989 to December 1993. Three hundred forty-two (92.7%) of the patients were ultimately free of stones after repetitive in situ SWL. The cumulative stone-free rates of the first, second, and third session were 64%, 81%, and 88%, respectively, and the increment in the cumulative stone-free rate thereafter with further repeated in situ SWL was minimal (p < 0.01). The cumulative stone-free rate at the third session was 89%, 87%, and 86% for proximal, middle, and lower ureteral stones, respectively (p > 0.05). The cumulative stone-free rate at the third session was 100%, 90%, 87%, 70%, 67%, and 50% for stones <5 mm, 6 to 10 mm, 11 to 15 mm, 16 to 20 mm, 21 to 25 mm, and >25 mm, respectively (p < 0.001). According to the radiopacity of the stone, the cumulative stone-free rate at the third session was 96% for stones with minimal opacity, 94% for those with moderate opacity, and 70% for highly opaque stones (p < 0.001). The cumulative stone-free rate at the second session was 100% without ureteral obstruction; 80% with mild, 57% with moderate, and 67% with severe obstruction; and 50% in patients with nonappearance of the kidney (p < 0.05). From these observations, it could be concluded that the factors influencing fragmentation were the size and radiopacity of the calculi and the degree of ureteral obstruction, whereas the location of the calculi did not influence the cumulative stone-free rate of repetitive in situ SWL. It is preferable to restrict in situ SWL to three sessions in patients with ureteral calculi refractory to shockwaves. An early change of treatment modality either to ureteroscopic manipulation or to open surgery would be recommended if there were risk factors as listed above.