Experiments on the pig and the human to measure retraction of skin flaps are described. Animal experiments show that it is dependent on the original length of the flap, and on the original length to breadth ratio for flaps perpendicular to the tension lines. The most important factor is the position of the flap relative to the tension lines. Flaps perpendicular to these lines do not retract and may indeed become longer, whereas flaps parallel to these lines retract by about 30%.