Objectives. To assess the significance of asymptomatic residual stone fragments of less than 4 mm (clinically insignificant residual fragments [CIRFs]) after extracorporeal shock wave lithotripsy (ESWL).

Methods. Eighty-one patients were followed up for 6 to 60 months (mean 15) after ESWL to determine the fate of the CIRFs.

Results. Of the 81 patients, 6 were lost to follow-up, leaving 75 patients. During follow-up, fragments passed spontaneously in 18 patients, remained stable in 13 patients, and became clinically significant in 44 patients who developed one or more complications. For the latter patients, repeated ESWL was done in 16, percutaneous nephrolithotomy in 3, and ureteroscopic stone removal in 4 patients. The remaining 21 patients were treated conservatively with analgesics. We found that 53% of the CIRFs located in the pelvis passed spontaneously, and most of the CIRFs in caliceal location became clinically significant. Also, as the stone burden and number of stone fragments increased, the risk of CIRFs becoming clinically significant increased. The outcome was the same whether a metabolic abnormality was present or not, provided the patient received appropriate treatment. The clearance rate was highest in the first 6 months. Finally, as the duration of follow-up increased, the rate of complications increased.

Conclusions. Patients with residual stones after ESWL require close follow-up and timely adjuvant therapy. As the number and size of residual fragments increased, the risk of complications increased. A pelvic location was a favorable factor for spontaneous passage. Metabolic defects, if treated adequately, did not increase the regrowth rate. Although the complete clearance rate of CIRFs with repeated ESWL was lower than for the operative interventions, most patients improved with this modality.