Background: Primary open-angle glaucoma (POAG) is the most common type of glaucoma. Increasing age, elevated intraocular pressure (IOP), and a positive family history are risk factors for POAG incidence. Elevated IOP is a major risk factor for glaucoma development and progression. The mechanism for IOP changes with body position is not completely understood. Objective: The present study was aimed to investigate the effects of different body positions on IOP in patients with primary open-angle glaucoma. Patients and methods: This prospective study included a total of 60 glaucoma and glaucoma suspect patients and 30 normal subjects, attending at Aswan University Hospital. Subjects were divided into 3 groups (normal, glaucoma suspect and primary open angle glaucoma). Each subject underwent thorough ophthalmological evaluation including best corrected visual acuity, slit lamp biomicroscopy, tonometry, gonioscopy, and dilated fundus evaluation. The Perkin's tonometer was used to measure IOP in different sleeping positions (at zero ^o degree, at 30^o degree and 45^o) positions. In all these positions, baseline IOP as well as after 30 minutes IOP was measured. Results: Comparison of both age and gender across different groups showed that there were no statistically significant differences among them. In terms of the IOP, comparing all of the three groups at baseline showed that there was a statistically significant difference across groups (p < 0.0001).  There was a statistically significant reduction of IOP in 30 and 45 degree semi sitting positions compared to supine positions in all study groups. Conclusion: It could be concluded that certain sleeping positions appear to be associated with changes in intraocular pressure. It is essential not to overlook these short term fluctuations particularly in patients with primary open angle glaucoma as they can affect progression of the disease.