Small black dots at the root of sweat hair on legs may be caused by hyperpigmentation, lifestyle habits, pigmented moles and other reasons. 1. Hyperpigmentation: the skin color deepens due to the increase of pigmentation caused by various reasons. The main clinical manifestation is the deepening of skin color, which leads to the phenomenon of small black spots at the roots of sweat hairs on legs, or the appearance of spots, patches or plaques that are darker than normal skin color. 2. Lifestyle habits: may be due to the roots of the leg hair is not clean or cleaning is not in place, dust, dust, etc. attached to the roots of the sweat hair so that the pores are clogged, which can appear small black spots. 3. Pigmented nevus: it is a benign neoplasm composed of nevus cells and a common benign skin tumor. It is generally a patchy rash or papule with a diameter of <6mm, and can appear brown, tan, blue-black or black. In addition, it is recommended that patients with small black spots at the root of sweat hair on the legs go to the hospital for timely observation and treatment as prescribed by the doctor to avoid delaying the condition.