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Background: Interarm differences (IADs) ≥10 mm Hg in systolic blood pressure (BP) are associated with greater incidence of cardiovascular disease. The effect of ethnicity and the white coat effect (WCE) on significant systolic IADs (ssIADs) are not well understood., Methods: Differences in BP by ethnicity for different methods of BP measurement were examined in 770 people (300 White British, 241 South Asian, 229 African-Caribbean). Repeated clinic measurements were obtained simultaneously in the right and left arm using 2 BPTru monitors and comparisons made between the first reading, mean of second and third and mean of second to sixth readings for patients with, and without known hypertension. All patients had ambulatory BP monitoring (ABPM). WCE was defined as systolic clinic BP ≥10 mm Hg higher than daytime ABPM., Results: No significant differences were seen in the prevalence of ssIAD between ethnicities whichever combinations of BP measurement were used and regardless of hypertensive status. ssIADs fell between the 1st measurement (161, 22%), 2nd/3rd (113, 16%), and 2nd-6th (78, 11%) (1st vs. 2nd/3rd and 2nd-6th, P < 0.001). Hypertensives with a WCE were more likely to have ssIADs on 1st, (odds ratio [OR] 1.73 (95% confidence interval 1.04-2.86); 2nd/3rd, (OR 3.05 (1.68-5.53); and 2nd-6th measurements, (OR 2.58 (1.22-5.44). Nonhypertensive participants with a WCE were more likely to have a ssIAD on their first measurement (OR 3.82 (1.77 to -8.25) only., Conclusions: ssIAD prevalence does not vary with ethnicity regardless of hypertensive status but is affected by the number of readings, suggesting the influence of WCE. Multiple readings should be used to confirm ssIADs., (© The Author 2017. Published by Oxford University Press on behalf of American Journal of Hypertension, Ltd.)