Within the framework of the Swiss National Research Program for the Prevention of Cardiovascular Disease, blood pressure levels of 8140 adults were recorded in four Swiss cities. Repeated measurements yield lower results. The values increase with age and are higher in men than in women. Swiss citizens have higher mean values than foreigners, and values in the French-speaking cities are significantly higher than those of the German-speaking cities. The prevalence of hypertension also depends on sex and age. The prevalence of elevated values (less than or equal to 160/95 mm Hg) identified by measurement, standardized for sex, age and nationality, is 11.6%; if the known, treated hypertensives are included the prevalence is 14.2%. 55% of the hypertensives knew of their condition, 39% were under medical treatment and only 20% were well controlled (greater than 160/95 mm Hg). There were considerable differences between the two region; fewer of the hypertensives were known and fewer were under control in the French-speaking towns.