Radioisotope studies provide unique functional and reasonable anatomic information with little radiation exposure risk as compared to conventional radiology. They can be used in all pediatric age groups, in severely ill patients and also in those with renal insufficiency. The radiopharmaceuticals are not toxic, not hyperosmolar, do not provoke allergic reactions and do not cause or aggravate renal dysfunction. Renal radioisotope studies have pushed IVU (intravenous urography) to the background. Radiopharmaceuticals can be classified into two principal groups. The first group includes agents that are rapidly eliminated by the kidneys and they enable evaluation of renal function and urinary drainage. Output efficiency is another major advance in the diagnosis of obstruction and provides greater accuracy than F+20.