There are many causes of inner thigh pain, most of which are caused by injury to the muscles attached to the medial femur, but can also be caused by a build-up of lactic acid in the muscles as a result of heavy activity. Pain in the inner thigh can also be caused by disease, such as a tumour pressing on a nerve in the femur, which generates impulses from the nerve to the brain that produce pain. Therefore, if you experience pain in the inner thigh, you should seek early medical attention.