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Cystine stones are often highly resistant to shock wave lithotripsy (SWL), but it has been reported that cystine stones of “rough” morphology are actually quite susceptible to SWL. Based on the observation that rough cystine stones contain void regions that are visible by helical computed tomographic (CT) imaging, we hypothesized that the internal structure of cystine stones would correlate with the susceptibility of stones to SWL. Cystine stones with average diameters between 4 and 7 mm were scanned using micro and helical CT, classified morphologically according to published criteria, and broken in a research electrohydraulic lithotripter, with fragments sieved through a 2 mm mesh every 50 SWs. Stones with regions of low X-ray attenuation visible on helical CT required only 650 ± 312 SW/g for total comminution, while those that did not show CT-visible internal structure required 1,046 ± 307 SW/g …