A small left kidney cyst 0.4mm is a small cystic dilatation on the left side of the kidney, usually benign lesions are common. Kidney cysts on one side and in small numbers are usually simple renal cysts, which occur more often in adults and increase in incidence with age. Most renal cysts have no obvious symptoms and are detected through physical examination. This condition does not require special treatment, as long as regular follow-up. If the cysts increase in size progressively or related complications such as rupture and bleeding of the cysts, infection, pain, or suspicion of cancer occur, timely treatment is needed. For patients with renal cysts, it is recommended to go to regular hospitals for standardized treatment, which needs to be evaluated by specialists to formulate a treatment plan.