OBJECTIVE: assess how many patients with low ambulatory diastolic blood pressure (DBP) are not identified when relying on office DBP alone, and thus have 'masked diastolic hypotension'. DESIGN: cross-sectional, retrospective cohort study. SETTING: academic hospital. SUBJECTS: 848 patients treated for hypertension who received ambulatory blood pressure monitoring (ABPM). METHODS: cut-off value between on- and off-target systolic blood pressure (SBP): 140 mmHg. Cut-off for low office and/or ambulatory DBP: DBP ≤ 70 mmHg. 'Masked diastolic hypotension' was defined as office DBP > 70 mmHg and mean ambulatory DBP ≤ 70 mmHg. RESULTS: mean age of the sample was 60 ± 13 years, 50% was female, 37% had diabetes, 42% preexisting cardiovascular disease (CVD), mean office blood pressure (BP) was 134/79 mmHg. In all patients (n = 848), low office DBP was present in n = 84(10%), while n = 183(22%) had low ambulatory DBP. In all patients with normal-to-high office DBP (n = 764), n = 122(16%) had 'masked diastolic hypotension'. In this group, ambulatory DBP was 14-19 mmHg lower than office DBP. Patients with low ambulatory DBP were older, had more (cardiovascular) comorbidities, and used more (antihypertensive) drugs. Antihypertensive drugs were lowered or discontinued in 30% of all patients with 'masked diastolic hypotension' due to side effects. CONCLUSIONS: 'masked diastolic hypotension' is common among patients treated for hypertension, particularly in older patients with CVD (e.g. coronary artery disease, diabetes), patient groups in which the European Society of Cardiology/Hypertension guideline advises to prevent low DBP. Although it remains to be examined at which BP levels the harms of low DBP outweigh the benefits of lowering SBP, our observations are aimed to increase awareness among physicians.