During prolonged open-sky vitrectomy, fluid accumulates in the suprachoroidal and subretinal spaces and the resultant ballooning of the choroid and of the retina interferes with the surgery. Fluorosilicone oil (polymethyl-3,3,3-trifluoropropylsiloxane) with a specific gravity of 1.28 was tested for use as a temporary vitreous substitute during open-sky vitrectomy. After open-sky vitrectomy and bullous retinal detachment in pigmented rabbits, fluorosilicone oil flattened the retina completely in nine of nine eyes, whereas 1% sodium hyaluronate solution flattened the retina completely in only four of nine eyes. The flattening effect of fluorosilicone oil on the retina was also better than that of 1% sodium hyaluronate in an experiment with retinal detachment and giant retinal tear after open-sky vitrectomy.