Red blood on the thighs depends on where the red blood is located. If it is distributed in the calf area, it may be related to the venous circulation in the calf bilaterally, or it may be caused by venous reflux disorder. If it is distributed in the thighs, it may be due to local trauma, or due to other factors causing hemangioma or vasculitis. This is because redness is either caused by dilated blood vessels or due to localized vasculitis. And vasculitis can usually be divided into two categories, one is cold vasculitis and the other is vasculitis with heat-excited erythema. The former is usually due to a cold environment or is a complication of cold globulinemia; the latter is usually due to thermal effects, such as those of heat radiation.