BACKGROUND: This study investigated the relationship of ethnicity to the differences between blood pressure (BP) measured in a clinic setting and by ambulatory blood pressure monitoring (ABPM) in individuals with a previous diagnosis of hypertension (HT) and without a previous diagnosis of hypertension (NHT). METHODS: A cross-sectional comparison of BP measurement was performed in 770 participants (white British (WB, 39%), South Asian (SA, 31%), and African Caribbean (AC, 30%)) in 28 primary care clinics in West Midlands, United Kingdom. Mean differences between daytime ABPM, standardized clinic (mean of 3 occasions), casual clinic (first reading on first occasion), and last routine BP taken at the general practitioner practice were compared in HT and NHT individuals. RESULTS: Daytime systolic and diastolic ABPM readings were similar to standardized clinic BP (systolic: 128 (SE 0.9) vs. 125 (SE 0.9) mm Hg (NHT) and 132 (SE 0.7) vs. 131 (SE 0.7) mm Hg (HT)) and were not associated with ethnicity to a clinically important extent. When BP was taken less carefully, differences emerged: casual clinic readings were higher than ABPM, particularly in the HT group where the systolic differences approached clinical relevance (131 (SE 1.2) vs. 129 (SE 1.0) mm Hg (NHT) and 139 (SE 0.9) vs. 133 (SE 0.7) mm Hg (HT)) and were larger in SA and AC hypertensive individuals (136 (SE 1.5) vs. 133 (SE 1.2) mm Hg (WB), 141 (SE 1.7) vs. 133 (SE 1.4) mm Hg (SA), and 142 (SE 1.6) vs. 134 (SE 1.3) mm Hg (AC); mean differences: 3 (0-7), P = 0.03 and 4 (1-7), P = 0.01, respectively). Differences were also observed for the last practice reading in SA and ACs. CONCLUSIONS: BP differences between ethnic groups where BP is carefully measured on multiple occasions are small and unlikely to alter clinical management. When BP is measured casually on a single occasion or in routine care, differences appear that could approach clinical relevance.