To determine the effects of routinely selecting the nondominant arm for ambulatory blood pressure monitoring (ABPM) on estimates of patients' blood pressure (BP) and to evaluate the practise of using manual BP from one arm and ambulatory BP from the other on the estimation of white coat effect (WCE), an observational study was conducted in 10 volunteers, exhibiting an interarm resting clinic systolic BP (SBP) difference > or =10 mm Hg. The main outcome measures were: (i) average ambulatory SBP measured on right and left arm simultaneously during 24 h, and (ii) estimate of WCE derived, by current practise, as the difference between the referral clinic BP (the higher of the manual readings from both arms) and ambulatory non-dominant arm BP, contrasted with the WCE calculated as the difference between clinic and ambulatory readings from the same arm (the arm with the higher manual readings). The supine referral clinic SBP was 16+/-6 mm Hg higher in the right compared with the left arm. Average 24 h ambulatory SBP was 6+/-7 mm Hg higher in the right arm (range +17 to -3 mm Hg), P = 0.025. Diastolic BP measurements mirrored the systolic findings. One-third of the WCE, estimated by current practise, could be attributed to inconsistency in the choice of arm for BP measurement. Thus, inconsistency in the selection of arms for BP measurement, by different techniques, may confound estimation of patients' cardiovascular morbidity risk.