Possible causes of swelling and pain in the left kidney area: 1, lumbar muscle strain. After the strain can appear local lumbar area soreness and pain, especially when the lumbar activities, the pain is more intense and obvious, usually with the history of lumbar injury or long-term fatigue history of the lumbar muscle, etc. 2, kidney stones. After the appearance of kidney stones, discomfort such as swelling and pain in the kidney area may occur. If the stone does not cause complete obstruction of the kidney, colic does not usually occur, and may be accompanied by frequent urination, urinary urgency or hematuria in the naked eye.3. Hydronephrosis. Hydronephrosis can be caused by congenital diseases or acquired diseases. For example, congenital narrowing of the renal pelvis and ureteral junction or tumors and stones in the kidney cause compression and result in hydronephrosis. Hydronephrosis can have a slow process, so patients always feel painful distention in the lower back. If complete obstruction is not caused, it usually does not cause acute pain. If the left kidney swelling and pain is not relieved for a long time, ultrasound or CT examination of the kidney is recommended to rule out chronic obstructive factors and other diseases.