A 48-year-old female patient was seen in our institution complaining of sudden onset of swelling and tenderness in the left side of her neck. Computed tomography and magnetic resonance imaging revealed abundant effusions in this area. Since these narrowed the patient's airway, we performed incisional drainage and intra-operatively we observed a transparent, colorless lymph fluid that oozed from the operative field, which was mainly a fatty layer at the supraclavicular area. Dilated lymphatic ducts were seen along the carotid artery and internal jugular vein, but damaged lymphatic ducts and obvious causative disease structures like tumorous lesions were not seen. Idiopathic lymph leakage was diagnosed due to the serous discharge from the left side of the neck and no apparent causative disease. The patient was successfully treated with incisional drainage and dietary restriction. Lymph leakage in the neck is more common after trauma or surgery. To our knowledge, no similar cases have ever been reported.