The causes of the bag on the left side of the back of the head are local trauma, folliculitis, lipoma, etc. It is recommended that the patient go to the hospital to clarify the specific causes of the disease through the cranial brain CT, and carry out symptomatic treatment under the guidance of the doctor. 1. Local trauma: such as impact, hit, etc., if the scalp is not broken, just subcutaneous edema, patients can relieve the symptoms through local cold compresses. 2. Folliculitis: folliculitis is usually caused by Staphylococcus aureus infection, patients can be treated with erythromycin ointment under the guidance of the doctor. 3. Lipoma: Lipoma is generally a benign cyst composed of mature cells, if the volume of the cyst is small, it can be treated without special treatment, if the volume of the cyst is large, then you can go to the hospital for surgical treatment. The appearance of the left side of the back of the head should be timely consultation, standardized treatment, to avoid delaying the condition.