When silicone oil fills the vitreous cavity of the phakic eye or the entire aphakic eye, the refractive correction can be expected to change by 5-9 diopters and sometimes as much as 14 diopters. Aphakic eyes become less hyperopic when filled with silicone oil, whereas phakic eyes become more hyperopic when the vitreous cavity is filled with silicone oil. Previously emmetropic aphakic eyes that are filled with silicone oil have only 4-6 diopters of hyperopia, whereas the same eyes have 10-12 diopters of hyperopia before insertion or after removal of silicone oil. Phakic eyes develop 5-7 diopters of hyperopia when the vitreous cavity is filled with silicone oil, and this puts them in the same range as the silicone oil filled aphakic eyes. High myopia, incomplete silicone oil filling, and several other factors influence the final optical outcome. The vitreoretinal surgeon must anticipate significant changes in refractive error when silicone is introduced into an eye or removed from the eye. Careful retinoscopy and refraction is essential for obtaining optimal vision in these patients.