A dark mole on the lips may be caused by a pigmented nevus, or it may be caused by melanoma, hyperpigmentation-polyp syndrome and other diseases. 1. Pigmented nevus:The cause of pigmented nevus is not clear, mainly considered to be caused by congenital heredity and developmental defects. When it occurs on the lips, the patient will have black patches, yellowish brown patches and other symptoms, so the patient will have the manifestation of nevus on the lips. 2. Melanoma: the cause of melanoma is not yet clear, and it is considered to be caused by malignant changes of multiple nevi and dysplastic nevi. The main symptoms of patients are that the area of the original nevus becomes bigger, and new pigment spots appear on the skin, so the patients will have the phenomenon of nevus on the lips. 3. Hyperpigmentation – polyp syndrome: the cause of hyperpigmentation – polyp syndrome is not clear, usually due to heredity. Patients will have hyperpigmentation, bowel sounds, intestinal polyps and other manifestations, so the patient will have the symptoms of nevus on the lips. When patients present with symptoms of nevus on lips, they should seek prompt medical attention for clear diagnosis and treatment.