Twenty-four-hour ambulatory blood pressure measurements were obtained during usual daily activities from two groups of normotensive adolescents: Cases had at least one hypertensive parent, and controls had both parents normotensive. Automatic measurements were programmed at 7.5-minute intervals. For each subject, within each of three time periods, the data were summarized by descriptors of the frequency distribution and the frequency spectrum. The time periods were sleep, school hours, and other waking hours. Regression analyses examined the relation between the blood pressure monitoring descriptors and case-control status adjusted for non-blood-pressure variables found to relate to case-control status. In stepwise logistic regression analysis with case-control status as the dependent variable, it was found that the mean level of diastolic pressure during school hours was significant, whereas the standard resting measurement was not. In linear regression analyses with blood pressure monitoring descriptors and standard resting measurements as dependent variables, several monitoring descriptors were found to be significantly related to the interaction between case-control status and certain variables that can affect blood pressure, such as weight and the presumed stress of school; no such relations were found for the standard resting measurements. These findings demonstrate the superiority of the ambulatory monitoring variables over the standard resting measurements in studying case-control differences.