In 78 of 103 patients with various vitreo-retinal pathology a peculiar photopsy has been revealed, which was called by the authors photopsy in a form of "side lightning". It appeared as bright arc-shaped light flashes in the periphery of the temporal part of visual field. It is stated that this symptom characterizes posterior detachment of the vitreous, which, in its turn, accompanies many vitreo-retinal diseases. Among 78 patients, 14 had retinal detachment, 52--central vitreo-retinal fibroplastic syndrome, 10--idiopathic macular apertures. In all cases photopsy in a form of "side lightning" was one of leading subjective symptoms. Correct assessment of this symptom favoured a timely diagnosis of vitreo-retinal pathology.