BACKGROUND: Patients with elevated clinic blood pressure and normal ambulatory blood pressure have a better prognosis than patients with sustained ambulatory hypertension, and may not have to be treated with antihypertensive drugs. On the contrary, current guidelines emphasize repeated clinic blood pressure measurements for the initiation of antihypertensive therapy. OBJECTIVE: To examine the relationship between ambulatory blood pressure at baseline and clinic blood pressure after 6 months of follow-up in untreated hypertensive patients, and the relationships of these pressures with the subsequent incidence of cardiovascular events. METHODS: Patients who were > or = 60 years old, with systolic clinic blood pressure of 160-219 mmHg and diastolic pressure < 95 mmHg, participated in the Systolic Hypertension in Europe trial. The relationship between ambulatory blood pressure at baseline and clinic blood pressure after 6 months of follow-up was examined in 295 patients enrolled in the Ambulatory Blood Pressure Monitoring substudy and randomized to the placebo arm, and who were still on double-blind treatment and not taking other antihypertensive drugs after 6 months follow-up. RESULTS: Age averaged 70 +/- 6 years, 41% were men, and baseline daytime ambulatory blood pressure was 152 +/- 16/84 +/- 10 mmHg; clinic blood pressure decreased from 173 +/- 10/86 +/- 6 mmHg at baseline to 163 +/- 20/85 +/- 9 mmHg at month 6. Systolic daytime ambulatory blood pressure at baseline and systolic clinic blood pressure at month 6 were considered normal if < 140 mmHg. Of the 74 patients with normal systolic daytime ambulatory blood pressure at baseline, only seven (9.5%) had a normal systolic clinic blood pressure during follow-up. Conversely, of the 24 patients with normal follow-up clinic blood pressure, only seven (29%) had a normal systolic daytime ambulatory blood pressure at baseline. The incidence of cardiovascular events beyond the 6-month visit was significantly related to baseline ambulatory blood pressure but not to follow-up clinic pressure. CONCLUSIONS: Baseline daytime ambulatory blood pressure and follow-up clinic blood pressure do not identify the same patients for antihypertensive treatment. Baseline ambulatory pressure is a better predictor of cardiovascular events than follow-up clinic pressure.