Blood pressure (BP) is frequently measured in patients by noninvasive blood pressure (NIBP) monitors. Values obtained by oscillometric devices of different brands may appear in one patient's record as if they were interchangeable; their concordance, however, has not been established. In 25 patients with major depression who were treated with electroconvulsive therapy (ECT) BP was measured on either arm by devices manufactured by SpaceLabs (SpL, 12 patients, 182 data points) and Marquette (Marq, 13 patients, 193 data points), respectively, and comparisons were made with simultaneous measurements on the opposite arm by Dinamap 1846SX (DIN), during the awake state and at 1-min intervals up to 5-7 min after ECT. Because ECT is associated with an intense, but short-lasting hyperdynamic state, comparisons of BP values could be made over a wide range of pressures. Bland-Altman plots were constructed to show the distribution of pressure differences at all pressures. Agreements between two instruments were judged according to guide lines by the American Association for Advancement of Medical Instrumentation (AAMI). The standard deviation of the difference (SDD) between two DIN devices was 7 mm Hg for systolic (SBP) and 6.3 mm Hg for diastolic blood pressure (DBP), whereas mean differences were 0.9 and 0.2 mm Hg, respectively (P = not significant [NS]), thus showing reproducibility. Corresponding SDD values SpL versus DIN were 9.1 for SBP and 8.3 mm Hg for DBP, while the mean differences were 1.6 (P = 0.026) and 7.3 (P = 0.0001) mm Hg, respectively. Corresponding SDD values for Marq versus DIN were 11.8 and 9.7 mm Hg with mean differences of 0.8 (P = NS) and 0.3 (P = NS) mm Hg. Whereas SBP differences DIN versus DIN exceeded 10 mm Hg in only 10% of observations, they exceeded that threshold in 31% and 32% of observations for SpL versus DIN and Marq versus DIN, respectively. In view of the variability that exceeds the AAMI guidelines and the one out of three occurrence of individual SBP differences exceeding 10 mm Hg for comparisons of SpL or Marq versus DIN, measurements by these three oscillometric devices are not interchangeable.