The aim of this study is to assess the effect of hospital admission on 24-h ambulatory blood pressure (ABP) in hypertensive subjects. Treated or untreated hypertensive adults with open-angle glaucoma underwent inpatient and outpatient 24-h ABP monitoring in a random order 4 weeks apart. Awake ambulatory hours, awake in-bed hours and sleep hours were reported by participants. The nighttime-to-daytime ABP dip (%) and the sleeping-to-awake dip (ambulatory and in-bed) were determined using the two ABP recordings. A total of 40 subjects were analyzed (mean age 65.7 ± 8.4 (s.d.) years, n=19 men). Daytime systolic BP (SBP) was lower in the hospital than in the outpatient setting (mean difference 4.3 ± 10.4 mm Hg, P=0.01), as was the awake ambulatory SBP (mean difference 5.0 ± 11.1 mm Hg, P=0.008). No differences were detected in 24 h, nighttime or sleeping SBP or in any of the respective diastolic outpatient vs. inpatient ABP measurements. The nighttime SBP dip (vs. daytime) was larger in the outpatient setting (8.9 ± 7.5% and 5.2 ± 4.7%, respectively; P=0.003). Sleeping SBP dip (vs. awake ambulatory and awake in-bed) was also larger in the outpatient setting (11.1 ± 7.3 and 7.8 ± 5.9%, respectively; P=0.02) with no difference in diastolic ABP. These data suggest that inpatient 24-h ABP monitoring does not reflect the usual BP level during routine daily life, nor does it represent the usual diurnal pattern of an individual. Relying on the 24-h ABP monitoring performed in the hospital environment may lead to an underestimation of ABP and an overdiagnosis of non-dippers. Therefore, 24-h ABP monitoring for decision making regarding diagnosis and treatment of hypertension should be performed only in the routine daily conditions of each individual.