Purpose To assess the association of stereopsis with differences between eyes (better minus worse eye value) and the binocular value of visual acuity and contrast sensitivity, and to analyse binocular summation or inhibition phenomena owing to differences between eyes. Methods A cohort of 137 patients with bilateral cataracts (visual acuity of 0.3 LogMAR or worse in both eyes) was followed up through first-and second-eye cataract surgery. The patients were recruited from the ophthalmology departments of two teaching hospitals. Visual acuity, contrast sensitivity (monocular and binocular), and stereopsis were measured preoperatively, after first-and second-eye surgery. Multiple linear regression and local correlation analyses were used. Results Stereopsis was most strongly influenced by visual acuity in the postoperative period after first-eye surgery (standardized coefficients of 0.382 for difference between eyes and 0.356 for binocular visual acuity) and by contrast sensitivity in the postoperative period after second-eye surgery (standardized coefficients of 0.353 for the difference between eyes and À0.312 for binocular contrast sensitivity). After first-eye surgery, the correlation of the differences between eyes with stereopsis was stronger (between 0.4 and 0.5) when the differences were greater than 0.4 logMAR units for visual acuity. Slight binocular summation was found for contrast sensitivity. Conclusions When assessing the indication for and outcomes of cataract surgery, analysis of visual function should include measures of both eyes, rather than measures of the operative eye only, as differences between eyes (better minus worse eye value) may play an important role in binocular measures such as stereopsis.