Background & Purpose: Our Dual Monitor Protocol with postural challenges in normotensives (n=15), hypertensives (n=14) and alcohol-dependents (n=11) revealed that an auscultatory ambulatory blood pressure monitor (ABPM) was inaccurate and unreliable, misclassifying 70% or more of hypertensives. Our goal was to apply the same procedures to determine the accuracy and reliability of oscillometric Oscar 2 and SpaceLabs 90207/90217 ABPMs. Methods: Two observers (O1, O2) using a mercury (Hg) column and ThinkLabs digital stethoscope assessed simultaneous, same arm BPs in 10 subjects (8 normotensives, 2 labile hypertensives) seated in the lab. ABPMs measured in triplicate simultaneous, opposite arm BPs on both arms alternating with Hg column BPs. Eight of 10 subjects were exposed to additional postural challenges in the lab, while four of 10 performed repeated 24-hr ABPM. Results: In the lab, the Oscar overestimated the observers’ systolic (SBP) by ~ 9 mm Hg (8.8, 95% CI: 6.9 - 10.1; 125.1 vs 116.3 mm Hg, P < 0.001), while the Spacelabs overestimated the observers’ SBP by ~ 5 mm Hg (5.4, 95% CI: 3.6 - 7.3, 121.7 vs 116.3 mm Hg, P < 0.001). Though there was a high degree of variability (both ABPMs, 15 mm Hg below to 23 mm Hg above O1O2), the average diastolic (DBP) difference was not statistically significant (Oscar vs O1O2: 69.7 vs 68.5 mm Hg, P = 0.27; Spacelabs vs O1O2: 69.9 vs 68.5 mm Hg, P = 0.23). For 24-hr ABPM in a normotensive, the Oscar had a similar average SBP (125.1 vs 125.6 mm Hg), but higher DBP (5.0 mm Hg, 67.6 vs 62.6 mm Hg, P < 0.001) and higher MAP (3.1 mm Hg, 86.7 vs 83.6 mm Hg, P < 0.001) vs observer-corrected values. For 24-hr ABPM in a hypertensive, the Spacelabs underestimated observer-corrected, 24-hr SBP, DBP and MAP by 5.2, 10.3 and 9.0 mm Hg (all P < 0.001). Both ABPMs were largely intolerant of motion, including stair climbing or walking and exhibited run-away inflations interpreted as not having exceeded SBP. Conclusions: While ABPMs may provide trends and predictions in population studies, we question ABPM accuracy and reliability in evaluating individual patients due to the high degree of variability. We find it unusual that approval protocols do not require postural testing. Further studies are needed to investigate the accuracy and reliability of oscillometric ABPMs.