Aims: To describe the evolution of ophthalmological and electrophysiological findings in monozygotic twin sisters with phosphomannomutase 2 deficiency (PMM2-CDG). Methods: A clinical ophthalmological examination with visual acuity measurement, fundoscopy and flash electroretinogram (fERG) was performed at the age of 4, 18 and 41 years. Results: Ophthalmic examination in both girls at the age of 4 years showed an alternating convergent squint and a saccadic pursuit, with visual acuity of 6/9 in both eyes (Ffooks symbols test). Fundoscopy revealed a normal aspect of the optic discs, narrowed blood vessels and a mild irregular pigmentation in the peripheral retina. Flash ERG in one girl showed a recognisable a, b1 and b2 wave, but with a reduction of the amplitude to less than 40% of the normal amplitude. In the other twin girl, the amplitude was more reduced, but a small b1 wave for the white flash was still noticeable. At the age of 18 years, vision had remained stable. Fundus examination revealed a pink aspect of the optic discs, with moderately narrowing of the vasculature and bone spicules in the mid peripheral retina. fERG showed obvious progression with a completely extinguished trace bilaterally. At the age of 41 years, vision had slightly diminished to 6/12 in both women. Fundoscopy and electroretinogram did not show any changes. Conclusions: Despite obvious deterioration of the fERG between the age of 4 and 18 years, the central vision showed only a minor decrease between the age of 18 and 41 years with still a good functional visual acuity.