Static pulmonary mechanics may not be the sole determinant of the functional residual capacity (FRC) in the chronic obstructive pulmonary diseases (COPD). To assess the relationship of FRC to pulmonary mechanics, pulmonary function tests such as spirometry, lung volumes, maximum expiratory flow volume curves, and single-breath diffusing capacity were related to compliance measurements in 65 patients with stable COPD. Compliance studies were examined over the range of tidal volume and at maximum lung inflation. The results showed that there was a poor correlation between the degree of hyperinflation at FRC and static compliance in the tidal range. However, there was a highly significant correlation between FRC and static lung mechanics at maximum lung inflation, as well as between FRC and diffusing capacity. Thus, in patients with stable COPD, FRC is a useful guide to static recoil properties of the lung as assessed at maximum lung inflation. Lung compliance measurement in tidal range may not reflect this relationship. An increase in FRC remains a useful index of pulmonary emphysema, even in the presence of chronic airway disease.