Obesity is very common (eg, one-half of WTC responders were obese at their initial examination), and obesity often causes dyspnea on exertion. A study of 100 obese patients (37% of whom reported dyspnea) noted that oscillometric abnormalities “were evident in essentially all subjects, thus confounding the ability of oscillometry to detect associated respiratory dysfunction independent of the effects of mass loading.”3 Spirometry and specific airway conductance (measured by body plethysmography) were normal. Thus, although it is easy to identify obesity in a patient, an abnormal FOT result in overweight patients may be falsely attributed to small airways disease causing their dyspnea. Note that in the pie charts Dr Berger and colleagues1 present, 50% of the symptomatic cases had abnormal oscillometry (but normal spirometry), but so did 22% of the control subjects. This suggests that about one-half of the abnormal oscillometry interpretations were false positive findings.