The most common cause of this condition is sebaceous cysts in the area behind the ear, which are caused by blocked hair follicles in the skin behind the ear. Patients may also develop secondary infection of the cyst, which may cause localized swelling and pain. Some patients may develop inflammatory enlargement of the lymph nodes in the area behind the ear, often associated with inflammatory infections of the surrounding skin and respiratory tract. Some patients with painless swollen lymph nodes behind the ear should be investigated for malignant tumors of the nasal cavity, sinuses, and nasopharynx, and for parotid tumors.