PurposeTo evaluates the role of measuring intraocular pressure (IOP) outside office-hour in primary adult onset glaucoma. MethodsThis retrospective study included 100 cases of primary adult onset glaucoma. IOP readings obtained with Goldmann applanation tonometry between 7 am and 10 pm were compared to office-hour readings. ResultsOne hundred patients were enrolled (mean age: 58.64±10.98 years) in the study. Overall, mean diurnal IOP was significantly higher than mean office IOP (p<0.05). Two-thirds of the patients had peak IOP measurements outside office-hour. Mean diurnal IOP fluctuation (7.03±2.69mm Hg) was significantly higher than mean office IOP fluctuation (4.31±2.6mm Hg) (p<0.003). There was a significant correlation between baseline IOP and fluctuation in IOP (r=0.61, p<0.001). ConclusionThe mean diurnal IOP and IOP fluctuations were higher than office-hour readings in patients with primary adult onset glaucoma. Diurnal monitoring may be particularly useful in patients with high baseline IOP.