PURPOSE: To assess the usefulness of power/color Doppler US in differentiating renal pseudotumors from isoechoic renal cell carcinomas (RCCs) based on characteristic vascular patterns. MATERIALS AND METHODS: Medical records of two hundred ninety two patients with histologically proven RCC by nephrectomy were retrospectively reviewed. Twelve patients who underwent conventional US and power/color Doppler US and those who with a mass less than 3 cm in size and isoechoic were selected. (group I) In addition, ultrasonographic findings of another 12 patients with renal pseudotumor initially diagnosed by US and followed up by US were also retrospectively reviewed. (group II) There was no change in size or echogenicity of the masses in the group II. The analysis and comparison of conventional US and color Doppler US findings between RCCs and renal pseudotumors were done with a special emphasis on the size, echogenicity, and vascular pattern of mass. Statistical analysis was done using Fisher's Exact Test, at 99% confidence level. RESULTS: On color Doppler US, all renal pseudotumors showed the same cortical vascularity as the adjacent renal parenchyma (Type 1). Among 12 patients with RCC, 5 RCCs showed a peripheral sweeping vascular pattern (Type 2) while 7 remaining RCCs showed peripheral sweeping vessels with fine penetrating branches (Type 3). There was a statistically significant difference between two groups (p<0.01). CONCLUSION: Power/color Doppler US showed typical vascular patterns in both pseudotumors and RCCs. These findings on power/color Doppler US could improve the diagnostic accuracy in differentiating renal pseudotumors from RCCs.