The presence of pulsus paradoxus (PP) in 13 episodes of status asthmaticus in 12 children, ages 13 months to 15 years, was compared sequentially to a clinical score, peak expiratory flow rate (PEFR), heart rate, arterialized capillary pH, carbon dioxide pressure (PCO2), and the ratio of inspired oxygen to oxygen pressure (FIO2)/PO2) during the first 48 hours following admission. There was a significant correlation (P less than .01) between the presence of a PP (greater than or equal to 5 mm Hg) and the clinical score (r = .79), PEFR (r = .55), and heart rate (r = .49). This was particularly striking when the PP was greater than or equal to 20 mm Hg. There was no significant correlation between the mean PP and the PCO2 or FIO2/PO2 ratio. However, a mean PCO2 exceeding 40 mm Hg was associated with a highly significant (P less than .005) difference in mean PP (22.2 mm Hg) compared to the mean PP (12.2 mm Hg) when the PCO2 was below 40 mm Hg. Although the PP technique can easily be learned by physician and nursing personnel, there are potential problems. The difficulties in children are compared to those in adults. The PP is a valuable clinical tool in assessing the severity of airway obstruction in status asthmaticus. The presence of a PP, particularly greater than 20 mm Hg, is associated with moderate to severe airway obstruction. In conjunction with the overall clinical status of the patient and frequent blood gas determinations, the PP allows for better evaluation of the patient with status asthmaticus.