The objective of this study was to determine the predictive value of the renal resistive index (RI) and cystatin C values in serum (SCys) and urine (UCys) in the development of acute kidney injury (AKI) in critically ill patients with severe sepsis or polytrauma. This was a prospective, double-center, descriptive study. There were 58 patients with severe sepsis (n= 28) or polytrauma (n = 30). Renal resistive index, SCys, and UCys were measured within 12 h following admission (day 1 [D1]) to the intensive care unit. Renal function was assessed using the AKI network classification: On day 3 (D3), 40 patients were at stage 0 or 1, and 18 were at stage 2 or 3. Patients with AKI stage 2 or 3 had significantly higher RI (0.80 vs. 0.66, P < 0.0001), SCys (1.23 vs. 0.68 mg/L, P = 0.0002), and UCys (3.32 vs. 0.09 mg/L, P = 0.0008). They also had higher Simplified Acute Physiology Score II, arterial lactate level, and intensive care unit mortality. In multivariate analysis, an RI of greater than 0.707 on D1 was the only parameter predictive of the development of AKI stage 2 or 3 on D3 (P = 0.0004). In the subgroup of patients with AKI stage 2 or 3 on D1, RI remained the only parameter associated with persistent AKI on D3 (P = 0.016). In multivariate analysis comparing the predictive value of RI, SCys, and UCys, RI was the only parameter predictive of AKI stage 2 or 3 on D3. Renal resistive index seems to be a promising tool to assess the risk of AKI.