Eighty excised human lungs were placed in a volume-displacement plethysmograph, and expiratory pressure-volume (PV) data points were obtained between transpulmonary pressures of 30 and 0 cmH2O. An exponential function of the form V = A - Be-KP was fitted to the PV data points (V is the volume at pressure P, A is the maximal lung volume at infinite pressure, B is A minus the intercept of the exponential on the volume axis, and K is a shape constant directly related to lung compliance). Thirty-two lungs were emphysema-free and 48 lungs had emphysema ranging from grade 2 to grade 80. In the emphysema-free lungs the exponential fit was excellent in terms of r2, but systematic deviations from the PV data occurred in both young and old lungs. Both K and B/A were significantly age related (P less than 0.01). In the emphysematous lungs the exponential fit was equivalent to that in the emphysema-free lungs although, again, systematic deviations from the PV data occurred. With age constant, K (P less than 0.01) but not B/A was significantly correlated with the emphysema grade. However, the overlap with values of K in the emphysema-free lungs was too great for K to be useful in the diagnosis of emphysema.