What ' s known on the subject? and What does the study add?Stone density on non-contrast computed tomography (NCCT) is reported to be a prognosis factor for extracorporeal shockwave lithotripsy (ESWL). In this prospective study, we determined that a 970 HU threshold of stone density is a very specifi c and sensitive threshold beyond which the likelihood to be rendered stone free is poor. Thus, NCCT evaluation of stone density before ESWL may useful to identify which patients should be offered alternative treatment to optimise their outcome. OBJECTIVE• To evaluate the usefulness of measuring urinary calculi attenuation values by non-contrast computed tomography (NCCT) for predicting the outcome of treatment by extracorporeal shockwave lithotripsy (ESWL). PATIENTS AND METHODS• We prospectively evaluated 50 patients with urinary calculi of 5 -22 mm undergoing ESWL.• All patients had NCCT at 120 kV and 100 mA on a spiral CT scanner. Patient age, sex, body mass index, stone laterality, stone size, stone attenuation values (Hounsfi eld units [ HU ] ), stone location, and presence of JJ stent were studied as potential predictors.• The outcome was evaluated 4 weeks after the ESWL session by NCCT.• ESWL success was defi ned as patients being stone-free (SF) or with remaining stone fragments of < 4 mm, which were considered as clinically insignifi cant residual fragments (CIRF). RESULTS• Our survey concluded that 26 patients (52%) were SF, 12 (24%) had CIRF and 12 (24%) had residual fragment on NCCT after a one ESWL treatment.• Stones of patients who became SF or had CIRF had a lower density compared with stones in patients with residual fragments [ mean ( SD ) 715 (260) vs 1196 (171) HU, P < 0.001 ] .• The Youden Index showed that a stone density of 970 HU represented the most sensitive (100%) and specifi c (81%) point on the receiver-operating characteristic curve.• The stone-free rate for stones of < 970 HU was 96% vs 38% for stones of ≥ 970 HU ( P < 0.001). A linear relationship between the calculus density and the success rate of ESWL was identifi ed. CONCLUSION• The use of NCCT to determine the attenuation values of urinary calculi before ESWL helps to predict treatment outcome, and, consequently, could be helpful in planning alternative treatment for patients with a likelihood of a poor outcome from ESWL. KEYWORDSshockwave lithotripsy , non-contrast computed tomography , urolithiasis Study Type -Therapy (prospective cohort) Level of Evidence 2b