Background: High blood pressure (BP) or hypertension is defined as a systolic and diastolic pressure over 140/90 mmHg. High blood pressure increases the risk for premature death, and previous research has shown that many patients do not reach targets and that there are differences between primary healthcare centers in the proportion of patient reaching targets. The reasons for these variations, however, are unknown. Aim: To investigate variations in blood pressure target achievement between primary care centers in Stockholm county and how different factors such as practice size, ownership, socioeconomic and antihypertensive drug treatment can influence this diversity. Method(s): This study was designed as a cross-sectional register study with a descriptive quantitative perspective. Data was collected from three sources: National Primary Care Quality register, Care Need Index for healthcare in Stockholm region and Stockholm County Council data warehouse VAL. The study included 179 out of all 227 primary care centers in the region. The proportion of all patients with hypertension reaching targets was assessed each year during 2019-2021, and correlations studied for potential predictors. Results: there was a variation between primary care centers in target blood pressure fulfillment, ranging from 22-66% during 2021, 23-63% during 2020 and 33-66% during 2019, respectively. There was no overall difference between public and private centers in the proportion of patients reaching targets, but a larger practice variation among private centers. No correlation was found between the other studied factors and target blood pressure fulfillment during 2021. Conclusion: There was a variation between primary care centers in the proportion of patients reaching blood pressure targets. Different practices may change ranking between years and other factors than practice size, ownership and socioeconomic appears to explain the variation.