A mass on the kidney is often found by ultrasound during a physical examination. What exactly it is, whether it is a benign or malignant mass, will depend on the clinical symptoms and further examination. Common benign masses include renal cysts and renal misshapen tumors, also called renal vascular smooth muscle lipomas. When the mass is relatively small, such as less than 5 cm, there are often no obvious symptoms. When the mass is larger than 5 cm, there is a possibility of hidden discomfort in the lower back and abdomen. More common malignant tumors of kidney masses include renal cancer and pelvic cancer, which often present as painless carnal hematuria, that is, just blood in the urine with no other uncomfortable symptoms, except that pelvic cancer may have hematuria much earlier. If a kidney mass is found, further CT or MRI examinations should be done, especially enhanced scans, and if necessary, puncture biopsies of the masses should be done and sent for pathological examination to determine the nature of the masses.