This study investigated components of blood pressure variability in elderly subjects with isolated systolic hypertension (ISH) using both ambulatory blood pressure monitoring (ABPM) and casual clinic blood pressure measurement. These were then used to determine sample size requirements for clinical trials of different designs. 2. Eleven elderly subjects not receiving antihypertensive medication were seen on four occasions at weekly intervals. On each occasion blood pressure was measured in the clinic and then for 24 h using a non-invasive ABPM device. Nested analysis of variance was used to calculate the 'between subject' and 'between subject within occasion' components of blood pressure variability. 3. Increasing the number of readings or occasions where measurement was performed in a parallel group trial only reduced the variability substantially when the number of subjects involved was less than 50. Use of a cross-over design substantially reduced the sample size required. 4. ABPM appears most useful as a strategy for reducing sample size in parallel group trials in ISH involving small numbers of subjects measured on one occasion.
