Complex renal cysts are cancerous or cystic kidney cancer. Complex renal cyst is a general term for a group of renal cystic lesions distinguished from simple renal cysts, which are completely independent cystic masses proliferating on the kidney, and the walls of the cysts may show fibrosis, vitreous degeneration, or calcification, and the clinical presentation and sonographic features are similar to those of some renal clear-cell carcinoma cystic lesions. However, complicated renal cysts have the possibility of cystic renal carcinoma, which needs to be evaluated by a combination of CT, MRI, and other examinations using the Bosniak classification to predict the possibility of malignancy, which is cystic renal carcinoma in some patients. Cystic renal carcinoma is a clinical and imaging concept that broadly refers to those renal cancers with cystic changes detected on imaging or during surgery. It includes cystic renal carcinoma, polycystic renal carcinoma, monocystic renal carcinoma, and simple renal cystic carcinoma. If complicated renal cysts are suspected or diagnosed, it is recommended to go to regular hospitals for comprehensive assessment of the condition, follow the doctor’s instructions to cooperate with the treatment, and choose the appropriate treatment plan to avoid delaying the condition.