In America, when a person is afflicted with baldness, the doctor takes hair from the back of his head, and plants it in the affected area. Is this practice permitted?
Yes, since this operation consists of returning Allaahs creation to what it was, to removing a defect, and not to beautify, or add to what Allaah Almighty created, then it does not fall under the category of changing the creation of Allaah. Rather it is a form of rectifying a deficiency and removing a defect. The Prophet (sallallaahu alaihi wa sallam) related a story about three men, one of whom was bald. One of them wanted Allaah Almighty to return his hair to him, the angel rubbed him, and Allaah Almighty gave him nice hair.
Shaykh Muhammad bin Saalih al-`Uthaymeen
Fatawa Islamiyah Vol. 8 Page 227