The aim of this prospective study was to determine the possible influence of the silicone oil tamponade after vitrectomy on the early intraocular pressure (IOP) elevation, which is a major risk factor for developing secondary glaucoma in patients with vitreal and retinal proliferative changes. The research included 110 patients which were allocated in three groups according to the medical history data. The surgical procedures were performed at the Eye Clinic, University of Zagreb School of Medicine. The control group comprised 40 patients who underwent vitrectomy with air or saline solution tamponade. The second group consisted of 40 patients with retinal detachment and proliferative retinopathy who had vitrectomy with silicone oil tamponade and the third group were 30 patients with diabetic retinopathy who underwent vitrectomy and tamponade with silicone oil. The intraocular pressure was measured and gonioscopy was performed in all patients one month before and after vitrectomy. The results showed that there is no statistically significant difference among IOP values before and after vitrectomy in the control group (p=0.104) as well as in the preoperative IOP values among all three groups of patients. The data analysis determined that in both groups of patients with silicone oil tamponade after vitrectomy, there is a statistically significant difference in IOP values one month after the surgical procedure (p=0.000). The mean IOP values in those patients a month after vitrectomy were significantly higher compared to the control group (p<0.05). Comparison of the IOP one month after vitrectomy between the patients with retinal detachment and those with diabetic retinopathy showed no statistically significant difference (p=0.331) but the qualitative analysis showed that the IOP one month after vitrectomy was 2 mmHg higher in the diabetic retinopathy group. The results suggest that there is no difference in angle width before and after vitrectomy among different groups of patients. Emulsified silicone oil was confirmed in 18% of patients in the retinal detachment group. In 17% of patients in the diabetic retinopathy group the emulsified oil was found in the angle, whereas a 10% of patients had neovascularization of the angle one month after vitrectomy. The IOP elevation in the early postoperative course may be caused by intravitreal instillation of the silicone oil after vitrectomy. Emulsification of the silicone oil may lead to the early IOP rise; especially in the diabetic patients with angle neovascularization which itself can additionally accelerate the development of the secondary glaucoma.