The authors report results of three cases of functional visual loss, where regular ophthalmic testing was not sufficient to confirm the diagnosis. In these cases, bilateral concurrent eye examination was performed using a head-mounted perimeter (imo), to facilitate the diagnosis. The imo is a head mounted perimeter developed during the advances in glaucoma diagnosis. It allows measurement of bilateral visual fields simultaneously, by randomly showing targets to each eye without the subject being aware of which eye is being tested. The results of the imo and those of Goldman perimetry or Humphrey Field Analyser II differed for all three subjects, which is indicative of potential functional visual loss. The authors conclude that the imo may become a standard assessment for diagnosis of unilateral functional visual loss in the future.