Objective:To evaluate the accuracy of oscillometric blood pressure measurement according to the relation between cuff size and upper-arm circumference in critically ill patients.Design:Prospective data collection.Setting:Emergency department in a 2,000-bed inner city hospital.Patients:Thirty-eight patients categorized into three groups according to their upper-arm circumference (group I: 18-25 cm; group II: 25.1-33 cm; and group III: 33.1-47.5 cm) were enrolled in the study protocol.Interventions:In each patient, all three cuff sizes (Hewlett-Packard Cuff 40401 B, C, and D) were used to perform an oscillometric blood pressure measurement at least within 3 mins until ten to 20 measurements for each cuff size were achieved. Invasive mean arterial blood pressure measurement was done by cannulation of the contralateral radial artery with direct transduction of the systemic arterial pressure waveform. The corresponding invasive blood pressure value was obtained at the end of each oscillometric measurement.Measurement and Main Results:Overall, 1,494 pairs of simultaneous oscillometric and invasive blood pressure measurements were collected in 38 patients (group I, n = 5; group II, n = 23; and group III, n = 10) over a total time of 72.3 hrs. Mean arterial blood pressure ranged from 35 to 165 mm Hg. The overall discrepancy between oscillometric and invasive blood pressure measurement was −6.7 ± 9.7 mm Hg (p< .0001), if the recommended cuff size according to the upper-arm circumference was used (539 measurements). Of all the blood pressure measurements, 26.4% (n = 395) had a discrepancy of ≥10 mm Hg and 34.2% (n = 512) exhibited a discrepancy of ≥20 mm Hg. No differences between invasive and noninvasive blood pressure measurements were noted in patients either with or without inotropic support (−6.6 + 7.2 vs. −8.6 + 6.8 mm Hg; not significant).Conclusion:The oscillometric blood pressure measurement significantly underestimates arterial blood pressure and exhibits a high number of measurements out of the clinically acceptable range. The relation between cuff size and upper-arm circumference contributes substantially to the inaccuracy of the oscillometric blood pressure measurement. Therefore, oscillometric blood pressure measurement does not achieve adequate accuracy in critically ill patients.