Small, preliminary studies have suggested that capnograms of obstructive lung disease (OD) exhibit a characteristic shape and that this shape may be correlated to changes in forced expiratory volume in 1 s (FEV(1)). We evaluated the association between capnograms and spirometry from subjects with OD with normal and restrictive lung disease (RD) subjects. The study was conducted in a prospective, nonrandomized manner using a convenience sample of 262 subjects presenting to a pulmonary function laboratory. Capnograms were recorded before pulmonary function testing. Subjects with OD had capnograms that were significantly different from normal and RD subjects. These differences were progressive, increasing with disease severity. The average take-off angle of the ascending phase for severe OD was 7.2 degrees less (95% confidence interval [CI]: 4.0, 10.4) than for normals. The average alveolar plateau elevation angle was 0.8 degrees more (95% CI: 0.14, 1.4) for moderate OD than for normals, whereas the average elevation angle was 3.6 degrees more (95% CI: 2.9, 4.3) for severe OD than for normals. Differences between OD capnograms and normal and RD capnograms, correlating to changes in FEV(1), were sufficiently large enough to suggest that the capnogram could be used to discriminate between OD and normal.