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To compare the reproducibility of ambulatory and office blood pressure readings in established hypertensive subjects when studies are repeated at extended time intervals.Twenty-five hypertensive patients (office diastolic blood pressure ≥90mmHg) who were off antihypertensive therapy for at least 4 weeks and had repeat office and ambulatory blood pressures at least 3 months apart under similar study conditions.On the same day, patients underwent office blood pressure readings measured by mercury column sphygmomanometry and then by ambulatory blood pressure monitoring. Ambulatory blood pressure monitoring was done for 24 h, and awake and sleep periods were divided according to a diary kept by each patient. A second study was performed in an identical manner at a mean±SD interval of 23 ±24 months (range 3–80, median 15). The agreement between studies was assessed by correlation coefficients, coefficients of variation and standard deviation of the differences (SDD).There were no significant differences in office, 24-h, awake and sleep mean blood pressures between the two studies. Mean 24-h systolic and diastolic blood pressures were 16 and 14mmHg lower, respectively, than office blood pressure values. Correlation coefficients were significantly higher for 24-h ambulatory blood pressure than office blood pressure, whereas the SDD between visits was significantly lower for 24-h ambulatory blood pressure than office blood pressure.These data demonstrate that long-term reproducibility of ambulatory blood pressure is superior to that for office measurement. One implication of this finding is that, in long-term clinical pharmacology trials utilizing ambulatory blood pressure, fewer subjects would be required than for studies that used office blood pressure end-points.