The left kidney cyst grows fast or not is related to the type of cyst, simple cysts generally do not grow very fast, if there is internal bleeding or infection in the cyst, there is a possibility that the cyst will increase in size in a short period of time. The causes of renal cysts are commonly congenital development, renal tubular diverticulum, etc. Simple renal cysts usually do not have obvious size changes for a long time and grow slowly, if the renal cysts are relatively small and do not affect renal function, there is no need for treatment, and follow-up with renal ultrasound observation can be performed in 6 to 12 months. If the renal cysts appear bleeding, infection and other conditions may appear a short period of time significantly increased, as well as a single cyst is larger than 5cm, may appear back pain, lumbar distension and other discomforts, and even affect the function of the kidneys, then you need to be through the cysts puncture and fluid extraction, surgical removal and other treatments. When left kidney cysts appear, it is recommended to consult a doctor in time to clarify the condition and further diagnosis and treatment under the guidance of a professional physician.