Studying renal function in patients with a solitary kidney undergoing SWL eliminates the compensatory effects of an untreated contralateral kidney and thus indicates any loss of function in the treated kidney. In 17 patients with a preoperative serum creatinine clearance of 1.5 mg/dl or less who underwent SWL to a solitary kidney, the glomerular filtration rate (GFR) was calculated by the formula of Cockcroft and Gault, which correlates significantly with measured creatinine clearance. A change of > or = 20% in the GFR was considered a clinically significant deterioration of renal function. In 12 patients with < 24 months' follow-up (mean 7.3 months) after SWL, there was no change in the estimated GFR, whereas 5 patients with > 24 months' follow-up (mean 36 months) had an average 22% reduction in the estimated GFR. This long-term effect is similar to the 29% long-term reduction in renal function recorded after percutaneous nephrolithotomy in solitary kidneys. Multiple stones, higher numbers of shock waves, and repeated SWL sessions correlated with decreases in GFR, but none of the differences was statistically significant. Like percutaneous nephrolithotomy, SWL results in a clinically significant long-term reduction in renal function.