Most hair transplant experts advise not to wear a hat immediately after hair transplant. This could compress, dislodge or move the grafts, which will compromise the success of the result. The last time they will advise to someone who had just gone through a hair transplant procedure is to wait 3 weeks to pass before they can wear a hat. This will give the transplanted grafts the chance to become a permanent part of your scalp. Enough time to ensure that the grafts are completely anchored. You can wear a loose hat though if it can't be avoided for you to go out, to keep out dust and other infection-causing particles from settling into the healing scabs on your scalp.