Background It is unknown whether “white coat” hypertension, also known as isolated clinic hypertension, shares similarities in pathophysiologic background with sustained hypertension. Therefore we evaluated 24-hour autonomic nervous function in sustained and white coat hypertension. Methods We selected 12 patients with sustained hypertension (clinic blood pressure ≥140/90 mm Hg and daytime blood pressure >135/85 mm Hg) and 12 patients with white coat hypertension (clinic blood pressure ≥140/90 mm Hg and daytime blood pressure <135/85 mm Hg) from patients undergoing ambulatory blood pressure monitoring and 12 normotensives for study inclusion. Groups were matched for age, sex, and body mass index and had similar dietary pattern and occupational status (civil servants with sedentary jobs). Subjects underwent noninvasive 24-hour monitoring of blood pressure, R-R interval of the electrocardiogram, body position, activity rate, and ambient temperature. Power spectral analysis of R-R intervals was performed with an autoregressive model to obtain the low-frequency component, the high-frequency component, and their ratio. Subjects also collected 24-hour urine samples for examination of norepinephrine and epinephrine excretion by high-performance liquid chromatography. Results Work and sleep time, body position, ambient temperature, and activity were not different among the groups. Daytime, nighttime, and 24-hour low-frequency/high-frequency ratios were significantly higher in patients with sustained hypertension than in patients with white coat hypertension (3.4 ± 0.45 vs 2.65 ± 0.45, 2.35 ± 0.60 vs 1.82 ± 0.45, and 3.04 ± 0.45 vs 2.4 ± 0.35, respectively, P < .05). Urinary norepinephrine excretion (53 ± 12 μg vs 29.5 ± 6 μg; P < .05) and vanillylmandelic acid excretion (4.45 ± 0.6 mg vs 3.1 ± 0.55 mg; P < .05) during the 24 hours were significantly higher in patients with sustained hypertension than in those with white coat hypertension. There was no difference between those with white coat hypertension and normotensives concerning the aforementioned parameters. Conclusions Our findings indicate whole-day sympathetic overactivity in sustained hypertension but not in white coat hypertension, suggesting that these conditions show some differences in pathophysiologic background. (Am Heart J 2000;140:672-7.)