BACKGROUND: Undiagnosed hypertension and uncontrolled blood pressure (BP) are common and contribute to excess cardiovascular morbidity and mortality. We examined whether BP control, changes in BP, and patient behaviors and attitudes were associated with a new hypertension diagnosis. METHODS: We performed a post hoc analysis of 323 participants from BP-CHECK (Blood Pressure Checks for Diagnosing Hypertension), a randomized diagnostic study of BP measuring methods in adults without diagnosed hypertension with elevated BP recruited from 12 primary care clinics of an integrated health care system in Washington State during 2017 to 2019. All 323 participants returned a positive diagnostic test for hypertension based on 24-hour ambulatory BP monitoring and were followed for 6 months. We used linear regression to examine the relationships between a new hypertension diagnosis (primary independent variable) and differences in the change in study outcomes from baseline to 6-month. RESULTS: Mean age of study participants was 58.3 years (SD, 13.1), 147 (45%) were women, and 253 (80%) were of non-Hispanic White race. At 6 months, 154 of 323 (48%) participants had a new hypertension diagnosis of whom 88 achieved target BP control. Participants with a new hypertension diagnosis experienced significantly larger declines from baseline in BP (adjusted mean difference: systolic BP, -7.6 mm Hg [95% CI, -10.3 to -4.8]; diastolic BP, -3.8 mm Hg [95% CI, -5.6 to -2.0]) compared with undiagnosed peers. They were also significantly more likely to achieve BP control by 6 months compared with undiagnosed participants (adjusted relative risk, 1.5 [95% CI, 1.1 to 2.0]). At 6 months, 101 of 323 participants (31%) with a positive ambulatory BP monitoring diagnostic test remained with undiagnosed hypertension, uncontrolled BP, and no antihypertensive medications. CONCLUSIONS: Approximately one-third of participants with high BP on screening and ambulatory BP monitoring diagnostic testing remained with undiagnosed hypertension, uncontrolled BP, and no antihypertensive medications after 6 months. New strategies are needed to enhance integration of BP diagnostic testing into clinical practice. REGISTRATION: URL: https://www.clinicaltrials.gov; Unique identifier: NCT03130257.