The pathological effects of focused electrohydraulic shock waves on renal parenchyma were studied using a porcine model. Testing was carried out using the Northgate SD-3 and Dornier HM-3 shock wave lithotripters. Pigs received 3,000, 5,000 or 6,000 shocks at energy levels equivalent to 18 to 20 KV on the Dornier HM-3. The animals were sacrificed one or four weeks post treatment and evaluated for renal injury. Kidneys were serially sectioned and injury volume calculations carried out. The predominant injury pattern was interstitial and perivascular fibrosis with chronic lymphoid infiltration. Dense areas of fibrosis ranged from less than 0.01% to 0.13% and from less than 0.01% to 1.04% of renal volume in those kidneys treated on the SD-3 and HM-3 respectively. Surrounding areas of perivascular and interstitial fibrosis intercalated with areas of normal appearing parenchyma were noted and were more extensive than the central scar. While the calculated volumes of parenchymal scarring are probably insignificant with respect to renal function, the surrounding areas of partial injury may be related to the development of hypertension.