Spirometer temperature was measured automatically during all years of the Lung Health Study. Short- and long-term changes in temperature were analyzed for the 23 dry-rolling-seal volume spirometers used at the 10 clinical centers involved in the study. Within-test-session spirometer temperature increased a mean of 0.3 degrees C, and as much as 3.0 degrees C during methacholine challenge testing. The maximal change in spirometer temperature during a test day exceeded 4 degrees C more than 5% of the time. Month-to-month changes of more than 15 degrees C were not uncommon. If ambient temperature had been assumed to apply to all maneuvers and used for BTPS corrections, FEV1, and FVC measurement errors of up to 6% would have occurred. When using volume spirometers, the temperature of air inside the spirometer should be measured accurately during each breathing maneuver.