With the widespread use of non-invasive diagnostic tools, such as abodminal ultrasound and computerized tomography, renal cysts are diagnosed with increasing frequency. In patients 50 years or older, simple renal cysts of various size may be found in nearly one third. Increasing frequency with age is clearly demonstrated. Two thirds of simple renal cysts are 2 cm or less in diameter. The average renal cyst needs about 10 years to reach 2 cm in size. Simple renal cysts (category I according to Bosniak classification) usually are asymptomatic, produce no harm to the kidney and require no treatment once diagnosed. However, an occasional eypanding cyst causes progressive obstruction to caliceal and pelvic outflow. There is a possible association between renal cysts and arterial hypertension. Renal cysts may produce segmental renal ischemia, and in turn activate the renin angiotensin system. Percutaneous cyst aspiration or surgical cyst removal could cause a fall in blood pressure. Bosniak suggested a classification in an attempt to sort out the different cases into nonsurgical (category I and II), and surgical ones (category III and IV). Borderline between cystic lesions type II and III is not clear-cut, but Bosniak type IV lesions are clearly cystic renal cell carcinoma.