A critical element in the study was a standardized surgical procedure for PVR. This surgical procedure, intended to relieve retinal traction with vitrectomy techniques, was followed by assessment of the relief provided by an intraocular air tamponade. The eye was randomized to silicone oil or gas only after completion of the entire surgical procedure to eliminate investigator bias that might develop through knowledge of the treatment modality. Patients were examined 5 to 14 days following the randomization and again at 1, 3, 6, 12, 18, 24, and 36 months after that date. Repeated surgery was permitted for either treatment modality. The Fundus Photograph Reading Center staff processed and analyzed photographs taken at all the clinics, graded the preoperative severity of PVR on the basis of baseline visit photographs, and confirmed the macular status at followup visits.