After a foreword concerning the present interest for the epidemiology of arterial hypertension mainly in relationship with the prevalence of the condition, its recognition in the general population and its systematic treatment, three separate chapters show the distribution of arterial blood pressure, its mean values and the prevalence of hypertension collected by three research groups: the Centre for Cardiovascular Diseases, St. Camillo Hospital, Rome, with 8 population groups (6929 subjects of whom 447 women, aged 20 to 64, belonging to the city of Rome and to other locations of 5 different regions); the II Medical Clinic, University of Padua, with 2 population groups (5852 men and women aged 20-64, belonging to a defined area of Veneto); the Research Group of the Roman Project of Coronary Heart Disease Prevention, with 2 population groups (2611 men aged 40-59, from Rome). The data provide a description of some characteristics of blood pressure and hypertension in different Italian areas and population groups and show the existence of large differences in the mean values of blood pressure and in the prevalence of hypertension, also within the country.