Sudden growth of many small black moles on chest may be caused by melanoma, pigmented nevus, age spots, patients should consult doctor in time, after clarifying the cause of the disease, treatment should be carried out. 1. Melanoma: melanoma is a tumor caused by malignant transformation of melanocytes, patients will have a lot of small black moles that grow suddenly and become bigger all the time, and there will be bleeding and itching, which can happen in chest, and it should be treated by surgery, radiotherapy and other treatments. 2. Pigmented nevus: a lot of small black moles suddenly grow on chest may also be caused by pigmented nevus, which has clear boundary with surrounding tissues and has no tendency of continuous growth, generally no special treatment is needed. 3. Age spots: if middle-aged and old people have a lot of small moles on chest suddenly, they should consider that they are caused by age spots, also known as seborrheic keratosis, small moles are round or ovoid, and they can be treated by laser and other methods. All the above operations should be carried out under the guidance of doctors.