Forty-six patients with Prader-Willi syndrome were examined to determine the incidence and character of ocular abnormalities. All patients met clinical criteria for this syndrome including infantile hypotonia, hypogonadism, truncal obesity, intellectual impairment, dysmorphic facies, and short stature. Thirty-two patients had best corrected visual acuities between 6/6 and 6/9 in each eye. Seven patients (15%) had myopia greater than -3.75 diopters. Nineteen (41%) patients had astigmatism of 1.25 diopters or greater. Amblyopia of strabismic, anisometropic, or ametropic etiology was present in 11 (24%) of the patients. Strabismus was present in 25 (54%) patients: 22 (48%) patients had esotropia and three (7%) had exotropia. Nine patients either received or required strabismus surgery. Thirty-three percent of the patients examined for iris transillumination defects had this finding. This study represents the first large series of patients with Prader-Willi syndrome to undergo detailed ophthalmologic evaluation. Recognition of this syndrome is important because of the high incidence of potentially treatable ocular problems.