Purpose <p>Sagging eye syndrome (SES) is the leading cause of acquired diplopia; however, there are no comprehensive reports on its treatment and prognosis in Japan. This study aimed to examine the disease classification of binocular diplopia in patients aged over 40 years old and examine the treatment methods and prognosis of SES.</p> Study design <p>Retrospective observational case series.</p> Methods <p>We reviewed the medical records of all new patients aged 40 years or older with binocular diplopia who visited the International University of Health and Welfare, Atami Hospital, between January 2020 and December 2022. Clinical causes of diplopia were analyzed based on age and sex; binocular alignment, treatment types, and post-treatment outcomes were documented for patients with SES, age-related distance esotropia, and cyclovertical strabismus.</p> Results <p>Among the 199 patients with binocular diplopia, SES was identified as the most common cause, affecting 95 (47.7%) individuals. Women comprised 52.6% of SES cases and 49% of non-SES cases (p=0.67). The mean age of onset was 71.2 ± 10.8 years for binocular diplopia, 76.2 ± 6.5 years for SES, and 66.6 ± 12.0 years for other etiologies (p&lt;0.001). SES treatments included surgery (48.4%, n=46), prism glasses (35.9%, n=34), and observation (15.8%, n=15). Among the 46 patients who underwent surgery and 34 who were prescribed prism glasses, 7 and 2 had residual diplopia, respectively, which resolved after additional treatment.</p> Conclusion <p>SES is a common cause of binocular diplopia in adults; however, effective treatment can successfully resolve diplopia in most cases.</p>