Aim: To determine the effects of diagnostic mydriasis using 1% tropicamide and 2.5% phenylephrine on the intraocular pressure of patients attending the Eye Clinic of University of Port Harcourt Teaching Hospital. Methods: This was a within-patient comparative hospital-based study in which the right eyes of 137 subjects requiring diagnostic mydriasis received 1% tropicamide and 2.5% phenylephrine. The left eyes served as control. Inclusion criteria were adults aged 18 years and older and eyes with normal open angles (Shaffer’s grade 3 or 4). IOP was measured with Perkins tonometer. Time of instillation of the first drop was taken as 0 and IOP was rechecked at 30, 45, 60, 90 and 120 minutes. Pulse rates and blood pressures were rechecked at 55minutes. Ocular discomfort grading and blurred vision assessments were done at 125 and 130 minutes respectively. Data were analyzed using the Statistical Package for Social Sciences (IBM SPSS) version 20.0 at significant level of p<0.05.