We have speculated about the influence of erroneously elevated volume ratios on the interpretation of published COPD outcome studies. For example, Fessler et al2 developed a model in which plethysmographically measured RV/TLC ratios predicted improvement in FEV1 following lung volume reduction surgery (LVRS). Because any plethysmographic error would be proportionately greater for RV, the RV/TLC ratio would be artifactually elevated. Among LVRS candidates in our sample, average plethysmographic RV/TLC was 64.9%, whereas average helium dilution RV/TLC was only 58.4%. When incorporated into the model of Fessler and colleagues, this difference in RV/TLC yields a twofold difference in predicted post-LVRS improvement in FEV1 (30% for plethysmographic vs 15% for He estimates). Also, the data of Casanova et al3 indicated an approximate 5% annual increase in mortality for each 1% decrement of IC/TLC ratio in COPD. Among our subjects, plethysmographic IC/TLC averaged 20.7%, whereas helium IC/TLC averaged 27.6%. This difference in the estimate of IC/TLC ratio yields an approximate 30% difference in predicted annual mortality. We believe these examples demonstrate potential consequences of plethysmographic error for predicting clinical outcomes and indicate the need to take a new look at an accepted measure.