Are blood pressure levels increasing in Denmark? The Copenhagen City Heart Study is a prospective cardiovascular population study designed to evaluate the incidence of and risk factors for cardiovascular disease. A random population sample comprising approximately 20,000 individuals was invited to participate. Blood pressure was measured, and information regarding the use of antihypertensive medication was collected in an initial survey during the period 1976-1978 (attendance rate 74%) and from a second survey during the period 1981-1983 (attendance rate 70%). A significant increase in systolic and diastolic blood pressure between survey 1 and survey 2 was found among both men and women greater than 40 years of age and not using antihypertensive medication. The increase in blood pressure in the follow-up survey could not be explained by changes in methods, changes in the prescription of antihypertensive medication, or selection bias. Factors associated with changes in systolic and diastolic blood pressure were examined by multiple linear regression analysis. Both increase in body mass index and increase in alcohol consumption were positively correlated with changes in systolic and diastolic blood pressure, while use of antihypertensive medication, a high value of body mass index at survey 1 and a high level of education were negatively correlated with changes in systolic and diastolic blood pressure. Female sex and advanced age were also negatively correlated with changes in diastolic blood pressure. Consumption of tobacco and alcohol, income and changes in consumption of tobacco were not significantly correlated with changes in systolic and diastolic blood pressure. 