BACKGROUND: The percentage of patients with office or white-coat hypertension has been reported in international studies to be 20-30% of the hypertensive population. These patients can be identified and distinguished from patients with established hypertension by ambulatory blood pressure monitoring (ABPM) or self-measurement. There is still no satisfactory explanation for the phenomenon of 'office hypertension' and there are no data available to show how the blood pressure behaviour of office hypertensives differs from that of normotensive subjects away from the physician's office or clinic environment. OBJECTIVE: To investigate the blood pressure behaviour of patients with office hypertension away from the clinical setting over a 24 h period and to compare it with that of normotensive subjects. DESIGN AND METHODS: The blood pressure and heart rate of 36 patients with office hyhpertension and 33 normotensive subjects matched for age and sex were measured in a hypertension outpatient clinic and over 24 h using ABPM. The ambulatory data were subjected both to conventional and to modern rhythm analysis. Urinary catecholamine excretion was measured as a marker of sympathetic activity. RESULTS: In the clinic, the patients with office hypertension had significantly (P < 0.01) higher blood pressure values (146/97 mmHg) than did the normotensive controls (128/81 mmHg). The conventional analysis as well as the rhythm analysis of the ABPM data revealed no difference between the two groups with respect to the 24 h, daytime or night-time mean values and SD. However, the rhythm analysis showed a higher maximum and a lower minimum value for systolic and diastolic blood pressures in the patients with office hypertension, resulting in a greater amplitude both of systolic and of diastolic blood pressure due to a significantly (P < 0.005 and P < 0.05) higher maximum minus minimum value (38/32 mmHg) compared with those of normotensive controls (29/28 mmHg). The early morning rise in systolic and diastolic blood pressures was significantly (P < 0.008 and P < 0.03) greater in the patients with office hypertension (11/9 versus 7/7 mmHg) and intersected the curve of the normotensive controls at approximately 0600 h. No significant differences in heart rate at any time were observed between the groups. The urinary excretion of noradrenaline and dopamine was significantly (P < 0.01 and P < 0.05) increased during daytime for the office hypertensives. CONCLUSION: Patients with office hypertension, who by definition do not yet have established hypertension, already exhibit abnormal regulation both of systolic and of diastolic blood pressure during the morning period and daytime, with a significantly greater early morning rise compared with normotensive subjects and a greater blood pressure amplitude (amplitude hypertension) due to lower blood pressure during night-time and higher blood pressure during the day with increased sympathetic activation. Office hypertension seems to be the earliest manifestation of hypertension.