A renal cyst is a small pocket filled with fluid and located in the kidneys. Also known as kidney cysts, many renal cysts are benign, and people can live out their entire lives without any knowledge of a cyst. In other cases, they are a cause for concern, and a doctor may recommend follow up treatment which can include surgery to remove the cyst when one is identified in the course of a medical examination and medical testing. Classically, renal cysts are identified on ultrasound, MRI, or another type of medical imaging study.

In the case of a simple cyst, the cyst shows up as a fluid-filled pocket, and if the cyst is aspirated, it can contain clear to yellowish fluid, sometimes mixed with blood. Complex cysts, on the other hand, have irregularities such as calcification, meaning that there is solid material inside the cyst, or they develop internal walls. A complex renal cyst is a cause for concern because it can be associated with the development of kidney cancer.