Sudden vision loss in a mucopolysaccharidosis I patient receiving enzyme replacement therapy: A 25-year-old female was referred for short stature and joint deformities. Except for previous corneal transplantation, her medical history was unremarkable. Initial physical examination revealed the presence of a coarse facies. short neck, kyphosis, restricted joint movements and deformities, and cardiac murmur besides a normal intellect. Urine glycosaminoglycan levels were high, and blood enzyme assay indicated significantly low alpha-L-iduronidase levels. Mucopolysaccharidosis I (MPS I) was diagnosed and prompted the onset of enzyme replacement therapy (ERT), which significantly improved articular complaints, while cardiac pathology remained stable. At the eighteenth month of ERT, sudden vision loss developed. She spontaneously recovered her vision in a month. MPS I is a progressive disease, in which tissue accummulation of heparan and dermatan sulphate result from defective activity or lack of alpha-L-iduronidase. ERT in MPS I usually presents favourable outcomes or at least stabilization of symptoms. This present case qualities as the first report of a NIPS I patient developing sudden vision loss under ERT. We suggest that further research studies are warranted for defining the efficiency and possible limitations of ERT.