INTRODUCTION: Increased use of nephron sparing surgery has revealed a small but significant percentage of benign tumors. Improved imaging techniques have aided in diagnosis, but are still unable to differentiate benign from malignant tumors. We sought to evaluate whether the intra-tumor Doppler flow pattern could predict the presence of renal cell cancer (RCC). MATERIALS AND METHODS: Standard grayscale ultrasound (US) and Power Doppler ultrasound (PDUS) were performed on 40 patients referred to our clinic for suspicious renal masses diagnosed by CT or MRI from December 2007 to May 2010. PDUS findings were used to classify tumors according to vascular patterns as proposed by Jinzaki et al, where pattern 0, 1, or 2 are considered diagnostic of benign renal lesions while patterns 3 and 4 predict malignancy. Clinical and pathological data were reviewed; ultrasound findings were correlated with histopathology. RESULTS: Of the 40 patients included for analysis, 13 underwent active surveillance, 24 underwent partial or radical nephrectomy, and 3 underwent ablative procedures. Twenty-seven (67.5%) patients had pathological specimens available for review, of which 22 patients had RCC and 5 had benign pathology. Intra-observer (kappa 0.46-0.70) and inter-observer (kappa 0.41-0.56) reliability were reasonable, but ratings didn't correlate with pathologic outcomes (all kappa < 0). CONCLUSIONS: Our results suggest that PDUS may not be helpful in the diagnosis of malignant renal masses detected by CT or MRI. Further studies are needed to elucidate a preoperative tool useful in diagnosing malignancy in renal masses.