Purpose To determine the effect of dilating drops on strabismus measurements in adults. Design Prospective cohort study. Methods Patients aged 18 and older with strabismus underwent a standard evaluation of ocular motility, stereopsis, and ocular alignment with alternate prism cover test by a certified orthoptist. After pupil dilation with 2.5% phenylephrine and 1% tropicamide, ocular alignment was remeasured in primary gaze at 6 meters, at 1/3 meter, and at 1/3 meter with a +3.00 by a second certified orthoptist, masked to the previous measurements. The primary outcome was the mean difference in the angle of horizontal and vertical deviations after dilation in prism diopters. Results A total of 55 patients were enrolled with a variety of diagnoses. For horizontal measurements the mean change was 0.54 prism diopters at 6 meters (95% confidence interval [CI] −0.36 to 1.43, P = .24), 1.67 prism diopters at 1/3 meter (95% CI −0.19 to 3.54, P = .08), and −0.05 prism diopters at 1/3 meter with a +3.00 add (95% CI −1.65 to 1.56, P = .95). The mean change in vertical deviation was 0.18 prism diopters at 6 meters (95% CI −0.19 to 0.56, P = .34), 0.57 prism diopters at 1/3 meter (95% CI 0–1.15, P = .05), and 0.47 prism diopters at 1/3 meter with a +3.00 add (95% CI −0.23 to 1.17, P = .18). Conclusions Pupil dilation does not meaningfully affect vertical or horizontal strabismus measurements in adults. Mild variability at near in younger patients is eliminated with the use of a +3.00 add.