Pulmonary mechanics were evaluated in 30 patients with acute myocardial infarction by measuring forced expiratory flow rates and total pulmonary resistance (Rt) with the oscillometric method at the resonant frequency of the chest (6-8) cycle/s. During the first 3 days after infarction, forced expiratory volume (FEV) and forced mid expiratory flow rate (FEF(25%-75%)) were 69% and 60% of predicted values, respectively. 10 or more wk later these values were 95% and 91%. Initially, Rt was 52% greater than predicted, but was only 4% greater 10 or more wk later. In 11 patients Rt was measured at both resonant frequency and at 3 cycle/s. Five of these patients had no clinical signs of heart failure, but nine had abnormally high values of pulmonary artery pressure, 'wedge' pressure and pulmonary extravascular water volume. All of these patients recovered. Initially, Rt at resonance was 50% and Rt at 3 cycle/s was 130% greater than predicted values. 2-3 wk later these figures were - 3% and + 6% of those predicted, respectively. At 10 wk or more, significant frequency dependence of Rt had disappeared (Rt at 3 cycle/s was 7% greater than Rt at resonance). Isoproterenol inhalation in six patients caused no change in flow rates, Rt at resonance, or Rt at 3 cycle/s. Rt at resonance and at 3 cycle/s, FEV and FEF(25%-75%) correlated significantly with the pulmonary vascular pressures. Patients with more marked arterial hypoxia and larger values for extravascular water volume had greater elevations of Rt and depression of FEF(25%-75%), but linear correlations were not significant. Clinical signs of congestive heart failure significantly correlated with a fall in FEV and FEF(25%-75%), the development of frequency dependence of Rt, and elevation of the pulmonary wedge pressure. The initial elevation of Rt and low flow rates indicate a modest degree of airway obstruction in acute myocardial infarction. Lack of response to isoproterenol suggests that bronchial muscular constriction is not a major factor. Frequency dependence of Rt accompanied by elevated pulmonary vascular pressures and extravascular water volume indicates that pulmonary congestion causes the development of uneven time constants in the airways. Vascular engorgement and interstitial edema from elevated vascular pressures causing narrowing of the peripheral airways and closure of collateral airways could account for the above findings.