We have evaluated the diagnostic value of deep breathing test (DBT) in detecting the patients with white coat hypertension on the outpatient basis. Thirty patients with untreated mild to moderate hypertension underwent 24-hour ambulatory blood pressure monitoring. Those who had a mean 24-hour blood pressure less than 135/80 mmHg were categorized as having white coat hypertension and the remainder were categorized as sustained hypertension. These two groups were compared with regard to the differences of office blood pressures before and after DBT (i.e., deep breathing 5 times for a minute in sitting position). Although the office systolic pressure significantly fell after DBT in both groups (p < 0.001 vs p < 0.05), there was no significant difference (p = 0.27) between the two groups. On the other hand, the office diastolic blood pressure significantly fell in the white coat hypertensives in contrast with no meaningful fall of it in the sustained hypertensives (p < 0.01 vs p = 0.66). At the cutoff level of -3%, -5% and -10% of the differences in office diastolic blood pressure before and after DBT, the sensitivity for the presence of white coat hypertension were 64.7%, 58.8% and 29.4%, respectively, and the specificity were 61.5%, 84.6% and 100%, respectively. These findings demonstrate that the deep breathing test is useful for identifying white coat hypertension in the outpatient clinic.