Fast-twitch extremity muscle fibers are preferentially affected in Duchenne's and Becker's muscular dystrophy. Since saccades are thought to be mediated by fast-twitch fibers, saccadic velocities would be expected to be decreased in these patients. Using infrared oculography, we found that the peak velocities of saccades in 3 patients with advanced Duchenne's or Becker's muscular dystrophy were normal. Clinical findings in 7 other patients with these forms of dystrophy were normal. This investigation is the first study of ocular motility in Duchenne's and Becker's muscular dystrophy. It demonstrates that extraocular muscle function is preserved and suggests that fast-twitch fibers in extraocular muscles possess properties that protect against degeneration.