Renal cell carcinomas (RCC) commonly occur among older people, and the prevalence of RCC is predicted to increase further in the future as the population ages. Early detection of RCC is significant because diagnosis in the symptomless stage obviously improves patient survival. However, the differential diagnosis of small intrarenal masses has recently become more difficult with easier detection of extremely small masses thanks to advances in screening techniques. Renal mass diseases include benign and malignant tumors. Angiomyolipoma (AML) is the most common among benign renal tumors, and renal cell carcinoma is the most common among malignant tumors. Meanwhile, renal mass lesions consist of solid and cystic masses. Almost all cystic masses are benign, but they can be malignant in some cases; therefore, a careful search for thickening of the site of involvement with US tomography is required. Moreover, intrarenal cystic masses may be renal vascular diseases such as aneurysm or aneurysmal type of arteriovenous fistula. Color Doppler US can be used for differentiation of a mass by confirming the presence of a vascular signal. Distinguishing RCC and AML from a hyperechoic small solid mass is needed in many patients, but differential diagnosis between them is mostly impossible even using color Doppler US with B-mode tomography. Contrast-enhanced US is helpful in these cases. A high rate of occurrence of RCC is well known in patients receiving maintenance hemodialysis, usually occurring due to acquired cystic disease of kidney (ACDK). B-mode and color Doppler are also ineffective for differentiating RCC and AML from those masses, and contrast US should be applicable in all patients. Information ranging from fundamental knowledge necessary for ultrasound diagnosis of the kidney to recent findings on contrast-enhanced US is summarized here.