Context: Discrepancies in ambulatory blood mercury sphygmomanometers pressure readings exist between automated blood pressure machines and manual mercury sphygmomanometers. For patients with elevated blood pressure, consistent blood pressure readings between these methods are important across all body mass index (BMI) levels. Objective: To determine the relationship between automated and manual blood pressure readings and the effect of BMI in patients with an elevated automated blood pressure reading. Methods: Automated and manual blood pressure readings (using the same arm) were collected from July 2014 to December 2016 across community-oriented primary care clinics in New York City for a retrospective medical record review. Automated systolic blood pressure (SBP) readings greater than 140 mm Hg or diastolic blood pressure (DBP) readings greater than 90 mm Hg were qualifying criteria for a manual blood pressure assessment. The difference in automated blood pressure readings relative to a manual blood pressure reading was assessed overall and for any relationship with BMI using linear regression and analysis of variance. Results: Data from 281 patients (166 [59%] women, 115 [41%] men; mean [SD] age of 57 [12.6] years) were assessed. For SBP, automated readings had an overall mean that was 8.0% greater than manual readings (P<.001). This relative difference decreased linearly by 2.7 points (95% CI, 1.0-4.4) for each 10-unit increase in BMI (P=.002). For DBP, automated readings had an overall mean that was 4.5% greater than manual readings (P<.001). This relative difference followed a quadratic relationship with BMI (P=.01), where the downward curve peaked at 6.6% (95% CI, 4.5-8.7) for a BMI of 35. When BMI was grouped into 4 categories, (normal weight, overweight, obese, and morbidly obese), morbidly obese patients had a smaller mean percentage SBP difference (4.3% [95% CI, 1.5-7.1]) compared with the other 3 categories (8.6% [95% CI, 7.2-9.9], P=.007). No relative differences between automated and manual methods for DBP were found among the BMI categories (P=.11). Conclusions: The current study found significant differences between automated and manual blood pressure readings. The relationship of these differences with BMI was also statistically significant, but their clinical significance remains inconclusive. Because manual blood pressure readings may have clinical value when evaluating or treating a patient with elevated blood pressure, better adherence to proper technique may improve accuracy.