BACKGROUND Relative afferent pupillary defect (RAPD) or Marcus Gunn pupil is a highly sensitive and significant objective clinical finding in eye examination. It can be detected by swinging flashlight test. Its presence indicates unilateral afferent sensory abnormality or an asymmetric bilateral disease. However, it can be determined even in an unconscious patient, requires no complicated gadgets, relies on clinical observation but can be a manifestation of a myriad of disorders affecting the eye or the brain. The goal of our study was to identify the aetiology of relative afferent pupillary defect and ascertain whether the grade of relative afferent pupillary defect correlated with the visual prognosis. METHODS This was a prospective observational hospital-based study. 25 consecutive patients who presented with a clinical diagnosis of relative afferent pupillary defect to the Ophthalmology Department of Kanyakumari Government Medical College in Tamil Nadu from February 2019 to January 2021 were included in the study. The patients were evaluated based on visual acuity, refraction, colour vision, slit lamp, fundus examination, intra ocular pressure (IOP) measurements & fields. Statistical analysis was done by trial version of statistical package for social sciences (SSPS) software. RESULTS Of the 25 cases analysed, 60 % (15) were male, 36 % (9) were females and 4 % (1) children. An analysis of the aetiology revealed that the optic nerve pathology was the predominant aetiology accounting for 60 % (15) of cases, followed by glaucoma 24 % (6) and retinal pathology in 16 % (4). Glaucoma though a bilateral disease has an asymmetrical presentation which led to RAPD. Grade of RAPD correlated well with the visual prognosis. CONCLUSIONS RAPD is a good and valuable clinical tool for any clinician, it is also a useful guide for assessing the management and response to treatment. KEYWORDS Relative Afferent Pupillary Defect, Optic Neuropathy, Retinal Detachment, Glaucomatous Optic Atrophy