The mechanism causing the tiger tail phenomenon in trichothiodystrophy (alternating light and dark banding of hair shafts when examined with polarized light) has yet to be explained with certainty. However, we propose a simple and easily tested hypothesis to explain its striking aspect. Although the hair shafts we obtained from patients with trichothiodystrophy were fairly straight, the cortical hair fibers were not. We noticed that these fibers undulate up and down (or back and forth), a feature that is easily observed because of melanin granules embedded in each fiber. The undulations correspond exactly to the banding seen with polarization. Therefore, the tiger tail phenomenon seen in trichothiodystrophy and other hair shaft disorders is caused by a regular undulation of hair fibers within the shafts. Normal hair shafts do not exhibit the phenomenon because the hair fibers are straight and parallel to the long axis of the hair. It is the regular undulation of fibers that changes the optical properties of the hair shafts and causes a predictable banding when the shafts are examined by polarized light.