Background: The U.S. Preventive Services Task Force recommends out-of-office blood pressure (BP) measurements before making a new diagnosis of hypertension, using 24-hour ambulatory monitoring (ABPM) or home BP monitoring. However, this approach is not common in routine practice. Blood Pressure Checks and Diagnosing Hypertension (BP-CHECK) is a randomized controlled diagnostic study comparing the accuracy and acceptability of clinic-, home-, and kiosk-based BP monitoring to ABPM for diagnosing hypertension. Methods: Within an integrated health care system, adults 18–85 years of age without diagnosed hypertension, on no hypertension medication, and with elevated BPs in clinic and at the baseline research visit are randomized to 1 of 3 regimens for diagnosing hypertension: 1) clinic BPs, 2) home BPs, or 3) kiosk BPs. All participants subsequently complete ABPM. Clinic BP patients are asked to make a clinic appointment to have their BP rechecked. Home BP patients are trained to use a validated home BP monitor and asked to check their BP twice a day, with 2 measurements each time for 5 days. Kiosk BP patients are trained to use a validated BP kiosk and asked to check their BP at least 3 more days with 3 measurements each time. All patients are asked to complete ABPM at 3 weeks. All BP data is being collected electronically (electronic health record, Bluetooth, web). The primary outcome is the accuracy of each method as compared to ABPM. Secondary outcomes include adherence to the assigned protocol and patient-reported outcomes related to diagnostic testing (discomfort, anxiety, perception of accuracy). Linear regression models will estimate the mean difference in systolic BP between the diagnostic and ABPM reference standard measures for each group. Results: At the time of abstract submission 427 of 510 (84%) of the expected total participants had been enrolled. Participants were 48% female, were 21% nonwhite or Hispanic, and had a mean body mass index of 30. Average baseline BP was 150 mmHg systolic and 88 mmHg diastolic. Enrollment ends in January 2019, and we expect to have preliminary primary accuracy outcomes at the time of the meeting. Conclusion: BP-CHECK will inform which hypertension diagnostic methods are most accurate, acceptable, and feasible to implement in primary care.