The Silicone Study was a randomized trial to investigate the relative merits of silicone oil or gas as tamponade modalities. All study patients underwent vitrectomy and were randomized intraoperatively either to silicone oil or to gas. Two groups of eyes were entered into the study: eyes that had not had a prior vitrectomy (Group 1) and those that had undergone previous vitrectomy outside the study (Group 2).