The relationship between kidney size and impaired renal function induced by shock-wave lithotripsy (SWL) was examined in 6- and 10-wk-old anesthetized pigs. Each pig received 2000 shock waves, 24 kV, or sham SWL to the lower pole calyx of one kidney. Bilateral GFR, renal plasma flow (RPF), and para-aminohippurate extraction was measured 1 h before and 1 and 4 h after SWL. The kidneys were then removed for morphometric analysis. Mean kidney weights were 66.1 ± 2.7 g (n = 9) and 103.1 ± 3.3 g (n = 8) in the SWL groups, and 60.1 ± 2.6 g (n = 9) and 82.3 ± 4.0 g (n = 9) in the sham-SWL groups. SWL-induced lesions occupied a significantly greater volume of the small kidneys (6.1 ± 1.7 vol % versus 1.5 ± 0.2 vol % in the large kidneys). RPF was significantly reduced by SWL in small and large kidneys, but to a significantly greater extent in small kidneys. RPF was also significantly reduced in the contralateral kidneys of both groups, but only at 1 h after SWL. SWL significantly reduced GFR to similar degrees in both kidneys of both groups, regardless of kidney size. Para-aminohippurate extraction was likewise reduced to similar degrees in both groups, but this effect was evident only in the SWL-treated kidneys, and only in the pole to which the shock waves had been applied. The injury induced by SWL affected a larger fraction of small kidneys than large ones, and the renal vasoconstriction induced by SWL was greatest in small kidneys.