BACKGROUND: The US Preventive Services Task Force recommends blood pressure (BP) measurements using 24-h ambulatory monitoring (ABPM) or home BP monitoring before making a new hypertension diagnosis. OBJECTIVE: Compare clinic-, home-, and kiosk-based BP measurement to ABPM for diagnosing hypertension. DESIGN, SETTING, AND PARTICIPANTS: Diagnostic study in 12 Washington State primary care centers, with participants aged 18-85 years without diagnosed hypertension or prescribed antihypertensive medications, with elevated BP in clinic. INTERVENTIONS: Randomization into one of three diagnostic regimens: (1) clinic (usual care follow-up BPs); (2) home (duplicate BPs twice daily for 5 days); or (3) kiosk (triplicate BPs on 3 days). All participants completed ABPM at 3 weeks. MAIN MEASURES: Primary outcome was difference between ABPM daytime and clinic, home, and kiosk mean systolic BP. Differences in diastolic BP, sensitivity, and specificity were secondary outcomes. KEY RESULTS: Five hundred ten participants (mean age 58.7 years, 80.2% white) with 434 (85.1%) included in primary analyses. Compared to daytime ABPM, adjusted mean differences in systolic BP were clinic (-4.7mmHg [95% confidence interval -7.3, -2.2]; P<.001); home (-0.1mmHg [-1.6, 1.5];P=.92); and kiosk (9.5mmHg [7.5, 11.6];P<.001). Differences for diastolic BP were clinic (-7.2mmHg [-8.8, -5.5]; P<.001); home (-0.4mmHg [-1.4, 0.7];P=.52); and kiosk (5.0mmHg [3.8, 6.2]; P<.001). Sensitivities for clinic, home, and kiosk compared to ABPM were 31.1% (95% confidence interval, 22.9, 40.6), 82.2% (73.8, 88.4), and 96.0% (90.0, 98.5), and specificities 79.5% (64.0, 89.4), 53.3% (38.9, 67.2), and 28.2% (16.4, 44.1), respectively. LIMITATIONS: Single health care organization and limited race/ethnicity representation. CONCLUSIONS: Compared to ABPM, mean BP was significantly lower for clinic, significantly higher for kiosk, and without significant differences for home. Clinic BP measurements had low sensitivity for detecting hypertension. Findings support utility of home BP monitoring for making a new diagnosis of hypertension. TRIAL REGISTRATION: ClinicalTrials.gov NCT03130257 https://clinicaltrials.gov/ct2/show/NCT03130257.