Pulmonary function has been evaluated in patients with acute uncomplicated myocardial infarction. Vital capacity,, flow-volume curves, oscillatory resistance and arterial oxygen tension have been measured within 24 hours of onset of symptoms and were repeated on day 4, 7 and 13 after infarction. In a mean interval of 6 months another measurement was done. Vital capacity was significantly reduced on the 4th day, arterial oxygen tension was also lowest on day 4, but no significant changes were seen. The expiratory flow data revealed a significant increase in the maximal expiratory flow as well as of maximal expiratory flow at 50% of the vital capacity in the second week after infarction. Oscillatory resistance was significantly increased on the first day and could be significantly reduced by inhalation of an anticholinergic broncholytic agent. These changes suggest that pulmonary function is reduced in patients with acute myocardial infarction, probably due to increase in pulmonary water and the maximal changes are present in the first week. The significant reduction of the oscillatory resistance from the first to the fourth day without significant changes of other parameters suggest an additional mechanism, which might be mediated by the vagal nerve.