Patients with oculocutaneous or ocular albinism have misrouting of optic fibers, with fibers from 20 degrees or more of the temporal retina crossing at the chiasm Instead of projecting to the ipsilateral hemisphere. Misrouting can result in strabismus and nystagmus. Because patients with the Prader–Willi syndrome may also have hypopigmentation and strabismus, we wondered whether they too might have misrouting of optic fibers. We therefore studied six patients with Prader–Willi syndrome selected for a history of strabismus, using pattern-onset visually evoked potentials with binocular and monocular Stimulation to look for evidence of misrouted retinal-ganglion fibers. Four had hypopigmentation, and three of these four had abnormal evoked potentials indistinguishable from those recorded in human albinos. The two with normal pigmentation had normal responses. These findings indicate that patients with Prader–Willi syndrome who have hypopigmentation have a brain abnormality characterized by mlsrouting of retinal-ganglion fibers at the optic chiasm — a finding previously reported only in forms of albinism. (N Engl J Med 1986; 314: 1606–9.), THE Prader–Willi syndrome is a congenital disorder that includes infantile hypotonicity; hyperphagia, with obesity developing after the neonatal period; hypogonadism; mental retardation; short stature; and small hands and feet. 1 2 3 4 5 Specific clinical characteristics of this syndrome are relevant to analyses of the optic systems. Hypopigmentation has recently been recognized as a feature of Prader–Willi syndrome, but with the exception of two case reports, patients with the syndrome who have hypopigmentation are not considered to have albinism. 3,6 Approximately 50 percent of the patients with Prader–Willi syndrome have hypopigmentation. Other features include a high incidence of strabismus (estimates vary from 40 to 90.