Objectives To evaluate the accuracy of US in calculating renal volumes and renal resistive index (RRI) that was obtained using a new method in patients with autosomal dominant polycystic kidney disease (ADPKD). Methods In this prospective study, US and MRI were performed in 57 patients with ADPKD (31 female and 26 male; age range, 19-79 years) between August 2017 and May 2018. The volumes determined using US and MRI were compared. The ellipsoid formula was re-evaluated using different multipliers. RRI was obtained 1.5-2 cm distal to the outlet of main renal arteries. The relationship between mean RRI, renal function tests, and kidney volumes and difference between mean RRI of ADPKD patients with and without renal failure were investigated using a two-sided independent samples t test and Pearson correlation test. Interobserver agreements for volume assessments and RRI measurements were determined. Results By changing the ellipsoid formula, a very good agreement was found (ICC 0.970 for the right kidney and ICC 0.973 for the left kidney). The mean RRI in the right renal artery was 0.61 +/- 0.07 and in the left renal artery 0.63 +/- 0.06. The mean RRI of ADPKD patients with renal failure was significantly higher than that of patients without renal failure (p = 0.005). There was a significant correlation between mean RRI and renal function tests. Conclusion The accuracy of the US in calculating renal volumes increases by adapting the ellipsoid formula. RRI may be used for the management of ADPKD independently of volumes.