The relationship between ambulatory blood pressure and mortality in a general Western population is unknown. Therefore, we conducted this prospective study of a random sample of 1700 Danish men and women, aged 41 to 72 years, without major cardiovascular diseases. At baseline, ambulatory blood pressure, office blood pressure, and other risk factors were recorded. After a mean period of 9.5 years, 174 had died: 63 were cardiovascular deaths. In multivariate proportional hazards models, adjusted for other risk factors of significance, the relative risk of cardiovascular mortality (95% confidence interval) associated with 10 mm Hg increments in systolic and 5 mm Hg increments in diastolic ambulatory blood pressure were 1.51 (1.28 to 1.77) and 1.43 (1.26 to 1.61). The corresponding figures for all cause mortality were 1.18 (1.06 to 1.31) and 1.18 (1.09 to 1.28). The relative risks of cardiovascular mortality were lower for office blood pressure, and office blood pressure did not predict all cause mortality. When ambulatory and office blood pressures were entered in the same multivariate models, only the ambulatory blood pressures were significant predictors of all cause mortality and cardiovascular mortality. The relationship between ambulatory blood pressures and risk of mortality was log-linear, with no indication of a threshold. The absolute risk of mortality was also dependent on age and smoking status, and an upper "acceptable" ambulatory blood pressure based on risk of mortality could only be defined when other risk factors were taken into account. In conclusion, ambulatory blood pressure provided prognostic information on mortality above and beyond that of office blood pressure.