1. The diagnosis of isolated systolic hypertension, diastolic hypertension and normotension in elderly subjects, as defined by casual office blood pressure measurement, was compared with 24 h ambulatory blood pressure monitoring using an Accutracker II. 2. Mean day-time ambulatory blood pressure monitoring underestimated the casual systolic blood pressure in all three clinical groups. Diastolic pressure was not underestimated to the same extent. 3. Ambulatory blood pressure monitoring best reflected casual blood pressure determination for normotensive subjects. In subjects with isolated systolic hypertension ambulatory blood pressures were only consistent with that diagnosis for 8% of the day time period. For 34% of the day time, their ambulatory blood pressures were consistent with diastolic/mixed hypertension. 4. It is concluded that isolated systolic hypertension may not be a sustained condition, but rather an isolated response to office measurement of blood pressure.