Strategies to reduce the burden of blood pressure attributable diseases require knowledge of secular trend in PBP and its determinants. The issues were investigated in the Copenhagen City Heart Study. The design of CCHS is a repeated measures study. Such designs are uniquely suited to studying changes of an outcome and what risk factors may be associated with that outcome. Repeated measures studies are very well suited for trend analysis by using mixed effect analyses. SBP decreased about 2 mmHg in 25 years. The risk factors age, gender and BMI were found valid as determinant factors for secular trends in SBP. In addition, the following factors were identified: household income and the interactions ''gender*age'' and ''survey*age''. The interaction ''gender*age'' stated that the difference between SBP in the two genders was great in the young individuals and diminished by age. The interaction ''survey*age'' stated that SBP in the young individuals decreased more with survey than SBP in the older individuals. Thus, the 20 years old subjects in survey 2, 3 and 4 have lower SBP than the 20 years old subjects in preceding surveys. The slopes were less steep in higher ages. In the group of elderly and old subjects the trend is partly explained by treatment bias because more and more subjects leave the untreated group and start treatment. The factor ''household income'' was significant only in the female population and stated that high-income women had lower SBP and a more beneficial secular trend in SBP than low-income women. Marital status, self-reported physical exercise and alcohol intake were not significant factors. A number of factors, that are interesting in relation to SBP, were not included in the CCHS and therefore not investigated. Among them are salt intake, childhood factors, genetic factors and the DASH diet. A survival study was performed to investigate the mortality rate in relation to SBP changes during the observation period. A Cox regression analysis was used in this study. The survival study demonstrated that SBP was a significant variable in survival models for all age groups. There was a decrease in mortality rate in young to middle-aged individuals. The mortality rate that is associated with a particular value of SBP did not change. Thus, it was concluded that SBP was as dangerous as it has always been and that the reduction in mortality rate was most pronounced in the age classes that also experienced the greatest reduction in blood pressure. During the observation period the number of treated individuals in the population increased from 6.5% to 18.1%. About 50% of the population was hypertensive (SBP ≥ 140 mmHg or treated with antihypertensive medication). The value of SBPtreated was used as an indicator for hypertension control in the treated population. Hypertension control is a collection of topics that includes guidelines, available medicine, physicians attitude towards hypertension treatment, systematic control, patient awareness and patient compliance. The analysis of trends in SPB in treated hypertensives showed that SBPtreated decreased 9.2 mmHg in 25 years. The result may be ascribed to improvements in treatment but may also be caused by a change in start-to-treat practice: If hypertensives start treatment at an increasingly lower SBPthreshold then SBPtreated will decrease without improvements in treatment. Therefore the start-to-treat practice was evaluated by SBPthreshold. A change in SBPthreshold was not observed. Thus, the 9.2 mmHg decrease in SBPtreated may represent improvements in treatment. ''Age'' was a significant factor for SBPtreated. This result demonstrated that elderly and old individuals were treated less successful than young and middle-aged individuals. Subjects diagnosed with ischemic heart disease constitute a group with a more advantageous slope than subjects with other diagnoses (stroke, IHD in combination with stroke, and hypertension alone). Self-reported physical exercise, gender, alcohol intake, household income and family structure were not significant as variables in the decreasing SBP among treated hypertensives. Thus, the papers in this thesis described SBP trends in the untreated and in the treated part of a population. Different patient-related factors were identified as determinant factors for trends in the two groups. The determinant factors are the explanatory variables most associated with trends in SBP. The determinant factors were different for the two groups (except for age).