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To evaluate the usefulness of urinary calculi attenuation values from non-contrast computed tomography (NCCT) in predicting the outcome of treatment by extracorporeal shock wave lithotripsy (ESWL).We evaluated 112 patients with solitary renal and upper ureteric calculi of 0.5–2 cm undergoing ESWL. All patients had NCCT at 120 kV and 240 mA on a spiral CT scanner. During each ESWL session 3000 shock waves were given to a maximum of 3.0 kV. A final X-ray of the kidney, ureters and bladder was taken 12 weeks after the last ESWL session. Fragments of ≤ 5 mm were regarded as clinically insignificant residual fragments (CIRF). The calculi retrieved were analysed by X-ray diffraction and the results assessed by comparing the mean density (as measured in Hounsfield units, HU) with the number of ESWL sessions and clearance.In all, 82 (76%) patients had complete clearance of stones and 26 (24%) had CIRF. There was a linear relationship between the calculus density and number of ESWL sessions required. Of patients with calculi of ≤ 750 HU, 41 (80%) needed three or fewer ESWL sessions and 45 (88%) had complete clearance. Of patients with calculi of > 750 HU, 41 (72%) required three or more ESWL sessions, and 37 (65%) had complete clearance. The best outcome was in patients with calculus diameters of < 1.1 cm and mean densities of ≤ 750 HU; 34 (83%) needed three or fewer ESWL sessions, and the clearance rate was 90%. The worst outcome was in patients with calculus densities of > 750 HU and diameters of > 1.1 cm; 23 (77%) needed three or more ESWL sessions and the clearance rate was only 60%. The calculus density was a stronger predictor of outcome than size alone.The use of NCCT for determining the attenuation values of urinary calculi before ESWL might help to predict the treatment outcome, and so might help in planning alternative treatment in patients with a likelihood of a poor outcome from ESWL.