To collect information about the retinal blood flow variations and other choroidal and retinal parameters during a prolonged effort such as marathon running.Non-randomized prospective cohort study.Patients were recruited through an information campaign at the Rothschild Foundation Hospital (Paris, France). A first visit (V1) was planned in the month before the marathon. All participants underwent blood pressure measurement, fundus photography, spectral domain-optical coherence tomography (SD-OCT) and OCT-angiography (OCT-A). A second visit (V2) was scheduled within one hour of crossing the finish line. The same tests were repeated, using the same equipment.Of the 31 runners who were included, 29 finished the marathon and attended V2. At baseline, various ophthalmological abnormalities were found in 45.2% of the 58 eyes, among which almost a third concerned the optic nerve and a quarter the pachychoroid spectrum. A significant decrease in retinal vascular plexus density was found between V1 and V2 (p0.01). While median macular and retinal nerve fiber layer (RNFL) thicknesses significantly increased after the marathon (p0.01), median choroidal thickness significantly decreased (p0.01). Both systolic and diastolic blood pressures significantly decreased (p0.01 and p=0.021 respectively).Prolonged physical effort impacts the structure and vascularization of the retina and the choroid. Hypoxia and dehydration due to such an effort may induce a low ocular blood flow rate resulting in a choroidal thinning, contrasting with a transient subclinical ischemic edema of the inner retina and optic nerve head.NCT03864380.