Clinic blood pressure (BP) is usually higher than daytime ambulatory BP in hypertensive patients, but some recent studies have challenged this view, suggesting that this relationship is strongly influenced by age. We used the Spanish ambulatory BP monitoring cohort to examine differences between clinic and daytime BP by age among 104 639 adult hypertensive patients (office systolic/diastolic BP ≥140/90 mm Hg or treated) in usual primary-care practice, across the wide age spectrum. To assess the impact of age, cardiovascular variables, and clinic BP on the clinic–daytime BP differences, we built multivariable regression models of the average BP differences, white-coat hypertension (high clinic BP and normal daytime BP), and masked hypertension (normal clinic BP and high daytime BP). In most patients, mean clinic BP values were higher than daytime BP at all ages. Some 36.7% of patients had white-coat hypertension (amounting to 50% at clinic systolic BP of 140–159 mm Hg) and 3.9% had masked hypertension (amounting to 18% at clinic systolic BP of 130–139 mm Hg). Age explained 0.1% to 1.7% of the variance of quantitative or categorical BP differences (P<0.001). Cardiovascular variables explained an additional 1.6% to 3.4% of the variance (P<0.001). Finally, clinic BP generally explained ≥20% more of the variance (P<0.01). In this large study in usual clinical practice, clinic BP misclassified hypertension status in >40% of patients. This misclassification was not importantly influenced by age but was more evident in patients with borderline/grade 1 hypertension. These findings reinforce the importance of ambulatory BP monitoring for defining BP status in routine clinical practice.