Black toenails in children may be caused by nevus of the nail, trauma, perinail warts and other diseases. 1. Nail nevus: Nail nevus is a kind of pigment cell nevus that grows in the nail root area, and its causes include drug stimulation, systemic pigmentation disease, heredity, hormone in the body and so on. Affected children will have blackened nail plate, and with the gradual growth of nail plate, regular black stripes can be formed. 2. Trauma: If the child’s toenails have recently been subjected to external violence, it will lead to local tissue blood circulation disorders, bruising, which can be manifested as blackened nails. 3. Periungual warts: Periungual warts are mainly caused by human papilloma virus infection. The actual warts can be formed in the nail bed or the skin around the nail, and they can be black spots, manifesting themselves as black toenails, and accompanied by localized pressure pain, tenderness. There are many causes of black toenails in children, parents need to bring their children to the hospital in time to find out the cause of the disease.