In 1039 subjects ambulatory blood pressure monitoring was performed to define threshold values, which are equivalent to the established limits of the office blood pressure. Mean values and proportions of elevated single readings were correlated to office blood pressure by regression analyses. To avoid impact of varying sleeping periods on 24 h blood pressure, only daytime readings were considered. Correlations between average daytime blood pressure and office blood pressure were linear: Mean daytime values of 135 mmHg (systolic) and of 84 mmHg (diastolic) were equivalent to the casual blood pressure limits of 140 mmHg and 90 mmHg. Correlations between percentages of elevated single readings and office blood pressure were nonlinear: Proportions of 25% systolic readings greater than 140 mmHg and of 17% diastolic readings greater than 90 mmHg were equivalent to casual blood pressure limits of 140 mmHg and 90 mmHg. On the basis of the regression equations, any result of ambulatory blood pressure recording during daytime can now be evaluated in terms of the established standards of office blood pressure.