In recent years, echographic studies of the kidney have improved radically due to new technologies which have recently become available. Among these, perhaps the most useful one is the ultrasonographic (US) procedure for the simultaneous laboratory and clinical workup of patients affected with acute nephropathic syndromes. However, traditional B-mode ultrasonography lack of sensibility and specificity in identifying and evaluating Acute Kidney Injury (AKI) is well known. Although the most objective measure in the study of different nephropathies remains by far the biopsy, several studies have indicated the usefulness of combining the B-mode ultrasound (US) with echo-color Doppler as a tool in determining intrarenal parenchymal arteries in the for differential diagnosis and predicion of clinical outcomes. In fact, the resistivity index (RI), determined by the formula: IR = (peak systolic velocity)--(end-diastolic or telediastolic velocity)/(peak systolic velocity) can be, after proper technical correction, easily measured at the level of the arcuate arteries or at the interlobar arteries. The final value is the average of 3-5 peaks, consecutively determined for each kidney at the upper pole, in the mesorenal area and also at the lower pole. The variation in normal IR values is < or = 0.70 with the difference diminishing progressively from segmental to interlobar vessels. Acute Kidney Injury (AKI) is perhaps one of the most important areas for the application of the Resistivity Index (RI). The differential diagnosis between prerenal AKI (which is functional and reversible if the cause of hypoperfusion is corrected) and renal AKI (which is organic and mainly caused by tubular necrosis (ATN) or acute interstitial nephritis) is facilitated by measurements of the RI, in addition to the normal clinical laboratory and clinical data. In fact, most pre-renal AKI patients show normal parenchymal vascular flow, with RI < 0.70, whereas those with AKI due to NTA have a reduced parenchymal perfusion, accompanied by elevated RI values, prior to any evidence of abnormal values of creatinine or of oligoanuria. Follow-up of patients with both renal and prerenal AKI by serial monitoring of RI during medical treatment of AKI shows a progressive reduction and ultimately the normalization of RI values of renal parenchymal vessels and often precedes the return to normal kidney function. In post-renal obstructive AKI patients, absolute values of RI > 0.70 on the obstructed kidney and a RI difference (deltaRI) between the two kidneys of > 0.06-0.08 are considered diagnostic of an obstruction. Elevated values of RI are also considered useful in the diagnosis of hemolytic-uremic syndrome (HUS) and are a significant predictor of prognosis: the normalization of IR precedes the return of normal renal functionality. Similarly, measurement of RI in patients with liver disease and normal renal function may help in early detection of latent hepato-renal syndrome. Although the IR is not, strictly speaking, a measure of renal function it may nevertheless be correlated with it especially if elevated arterial resistivity is accompanied by a reduction in renal function itself. Thus, IR may be considered a useful predictive index in specific clinical settings.