Determination of absolute lung volumes in patients is most reliable when measured with body plethysmography. Many laboratories use data obtained with a spirometer not directly linked to the plethysmograph to calculate total lung capacity (TLC) and residual volume (RV) from thoracic gas volume (Vtg) measured at functional residual capacity (FRC) in the plethysmograph. The reliability of these calculations depends on the stability of FRC between these separate devices. We examined the differences in TLC and RV values calculated with linked and unlinked spirometers in 220 patients and found them statistically significant (p less than 0.05). Additionally, differences exceeding the 95 percent confidence intervals for repeated, linked determinations occurred in more than 5 percent of patients. The large-volume differences in TLC were often associated with differences in expiratory reserve volume (ERV) in the opposite direction, suggesting a shift in FRC. However, clinical diagnoses were infrequently (4/220) altered by these differences, and recognition of the shift in FRC should further reduce this error. Therefore, the unlinked method appears acceptable.