Background: Isolated functional methionine synthase defi-ciency occurs in the cblE and cblG defects of methylcobalamin metabolism and is one of a number of c auses of severely elevated plasma homocysteine. Clinical features are predominan tly of a neurological nature but also include functional restriction of the visual system manifesting as loss of visual acui ty and nystagmus. As yet, the origin and pathogenesis of impaired vision have no t been explained. Materials and methods: We investigated a patientwhowas proven by complementation analysis in cultured fibroblasts to belong to the cblG comple mentation group. Ganzfeld electroretinograms (ERG)-and flash visual evoked pote ntials (VEP) were recorded over a period of 4 years. Results: Amplitudes of all International Society for Clinical Electrophysiology of Vision (ISCEV) standard responses were below normal. The greatest reductions were of rod response to 24 μV, of standard combined response (SC) b-wave to 120 μV, of oscillatory poten tials (OP) to 5 μV, of cone response b-wave to 35 μV, and of 30Hz flicker res ponse to 8 μV. Except for SC and cone a-waves at age 2.5 and 3.5 years, as wel l as cone b-wave at 3.5 years, amplitudes remained at a subnormal level at foll ow-up examinations. Implicit times were slightly prolonged (SC b-wave 6 ms, OP s 2 ms, cone b-wave 2 ms, 30 Hz flicker 4 ms) or fell within the normal range. Responses of the flash VEP were severely deformed but reproducible. Conclusions: This is the first report of detailed investigations of the visual system in a p atient with isolated methionine synthase deficiency. Reduced oscillatory potenti als suggest microvascular damage to the retina through homocysteine. Decreased p hotoreceptor function as well as ganglion cell loss as indicated by pathological flash VEPs may reflect a cytotoxic impact of homocysteine on neurons of the vis ual pathway. Background: Isolated functional methionine synthase defi-ciency occurs in the cblE and cblG defects of methylcobalamin metabolism and is one of a number of c auses of severely elevated plasma homocysteine. Clinical features are predominan tly of a neurological nature but also functional restrictions of the visual system manifesting as loss of visual acui ty and nystagmus. As yet, the origin and pathogenesis of impaired vision have no t been explained. Materials and methods: We investigated a patientwhowas proven by complementation analysis in cultured fibroblasts to belong to the cblG comple mentation group. Ganzfeld electroretinograms (ERG) -and flash visual evoked pote ntials (VEP) were recorded over a period of 4 years. Results: Amplitudes of all International Society for Clinical Electrophysiology of Vision (ISCEV) standard responses were below normal. were of rod response to 24 μV, of standard combined response (SC) b-wave to 120 μV, of oscillatory Except for SC and cone a-waves at age 2.5 and 3.5 years, as wel l as cone b -wave at 3.5 years, amplitudes remained at a subnormal level at foll ow-up examinations. Implicit times were slightly prolonged (SC b-wave 6 ms, OP s 2 ms, cone b-wave 2 ms, 30 Hz flicker 4 ms) Responses of the flash VEP were severely deformed but reproducible. Conclusions: This is the first report of detailed investigations of the visual system in ap atient with isolated methionine synthase deficiency. Reduced oscillatory potentials suggest microvascular damage to the retina through homocysteine. Decreased p hotoreceptor function as well as ganglion cell loss as indicated by pathological flash VEPs may reflect a cytotoxic impact of homocysteine ​​on neurons of the visual pathway.