Background: In the evaluation of patients with acute asthma, pulsus paradoxus (PP) is an objective and non-invasive indicator of the severity of airway obstruction. However, in children PP may be difficult or impossible to measure. Indwelling arterial catheters facilitate the measurement of PP, but they are invasive and generally reserved for critically ill patients. Objective: To determine the utility of the plethysmographic waveform (PPpleth) of the pulse oximeter in measuring PP. Methods: Patients from the pediatric intensive c are unit, emergency department, and inpatient wards of a tertiary care pediatric hospital were eligible for the study. A total of 36 patients (mean age [SD], 11.2 [4.7] years) were enrolled in the study. Pulsus paradoxus was measured using the traditional auscultatory (PPAUSC) method with a sphygmomanometer. Pulsus paradoxus was then measured using a blood pressure cuff observing for the disappearance and reappearance of the (PP pleth) on the pulse oximeter. Mean difference and 95% confidence intervals were calculated for each method. The 2 methods were also analyzed for correlation and agreement using the Pearson product moment correlation and a Bland and Altman plot. Results: Patients with status asthmaticus had higher PPAUSC and PPpleth readings compared with nonasthmatic patients. Pulsus paradoxus measured by plethysmography in patients with and without asthma was similar to PPAUSC readings (mean difference, 0.6 mm Hg; 95% confidence interval, -0.6 to 2.1 mm Hg). Individual PP pleth readings showed significant correlation and agreement with PPAUSC readings in patients both with and without asthma. Conclusion: Measurement of PP using the pulse oximeter-pulse plethysmographic waveform offers a simple and noninvasive method for evaluating patients with airway obstruction.