We performed color Doppler sonography in six patients with arteriovenous malformations of the kidneys. The diagnosis was established by angiography in all patients. Color Doppler sonograms were obtained at a large Doppler frequency-shift range (55 cm/sec of maximal average flow velocity at zero Doppler angle) to depict high-velocity blood flow in the malformation. In all patients, the malformations were seen as focal areas of flow, portrayed as a mixing of lighter colors. These were reflected by a rapid flow rate and marked tortuosity of the vessels. The sonograms showed a small peripheral malformation that was indistinct on selective renal angiography. However, flow in normal vessels grouped in the hilum obscured lighter-color flow of a small central malformation. In three patients who had total or partial ablation of the malformations with alcohol, follow-up color Doppler sonograms showed that the focal areas of flow, represented as mixing of lighter colors, disappeared or became smaller. This study shows that color Doppler sonography is a useful noninvasive procedure for diagnosing arteriovenous malformations of the kidney.