PURPOSE: To determine the vergence response, if any, in a homogenous group of patients suffering from strabismus and submitted to environmental induction of retinal disparity with respect to a control group of subjects. MATERIALS AND METHODS: A series of 13 subjects was included in the present study (5 normal subjects and 8 patients suffering from small angle esotropia). The patients were evaluated in an illuminated environment where they were told to stare at a target which subtended a visual angle of 20 degrees and, successively, in the dark using a luminous stimulus which subtended a visual angle of 3.5 minutes. The vergence movements were evaluated using a four diopter base-out prism; eye movements were recorded under conditions of both binocular and monocular vision. Ocular movements were recorded by means of a magnetic search coil; the amplitude and peak velocity of the vergence movements were evaluated by computer analysis. Statistical analysis was performed using the Multiple Anova test for repeated measures. RESULTS: The data obtained in the present study demonstrate that: 1) there is a significant difference between binocular and monocular vision in both groups of subjects; however, the ocular response in the patients affected by strabismus was significantly different from that in the control group of subjects; 2) during monocular vision in the dark, the response of the strabismic patients was highly variable; 3) for both groups of subjects no significant differences were observed in the vergence response when a large-angle stimulus was employed in light conditions nor when a small-angle stimulus was utilized in a dark environment.