We studied 573 patients with the Marfan syndrome for the presence of ocular misalignment, refractive errors, and amblyopia. A total of 110 patients (19.2%) had strabismus. Of 573 patients, exotropia occurred in 67 (11.7%), esotropia in 12 (2.1%), vertical deviations in eight (1.4%), and primary inferior oblique muscle overaction in three (0.5%). Of 67 patients with exotropia, 18 had anisometropia and 39 had amblyopia. Of the 12 patients with esotropia, four had amblyopia and none had anisometropia. Exotropia and esotropia are more common in patients with the Marfan syndrome than in the general population of the United States (P < .001 for exotropia and .05 < P < .10 for esotropia). Abnormal afferent visual inputs to cortical centers caused by ectopia lentis, craniofacial abnormalities, and mechanical and genetic factors may all contribute to the higher prevalence of strabismus in this disease.