Duplex Doppler ultrasound (US) was used in 21 consecutive patients with renal masses, including renal carcinoma (n = 9), metastases (n = 6), lymphoma (n = 4), angiomyolipoma (n = 1), and sarcoma (n = 1). Seven of the nine carcinomas had Doppler shifts of 4 kHz or more and were hypervascular or vascular. These seven masses had significantly higher (P less than .001) Doppler shifts than all the other malignant renal masses, including the two avascular renal cell carcinomas. Correlation with angiographic findings suggests that the high-frequency Doppler signals were associated with arteriovenous shunts. Detection of the high Doppler shift seems to be a promising step in differentiating hypervascular renal carcinomas from other renal tumors.