As part of an on-going population-based study of patients hospitalized with acute myocardial infarction (MI) in all 16 hospitals in the Worcester, Massachusetts Standard Metropolitan Statistical Area, temporal trends in the use of the pulmonary artery (PA) catheter were examined. Three thousand two hundred and sixty-three patients with validated acute MI during the calendar years 1975, 1978, 1981 and 1984 comprised the study population. There has been a consistent and significant increase in PA catheter use in patients with acute MI over time, from 7.2 percent in 1975, 13.8 percent in 1978, 14.8 percent in 1981 to 19.9 percent in 1984 (p less than .001). Ninety-six percent of patients undergoing PA catheter investigation had either congestive heart failure (CHF), hypotension or cardiogenic shock. For the combined time periods, the in-hospital case fatality rate (CFR) for patients in CHF with a PA catheter was 44.8 percent compared to 25.3 percent for patients without a PA catheter (p less than .001). For patients with hypotension and a PA catheter, in-hospital CFR was 48.3 percent compared to 32.2 percent for hypotensive patients not receiving a PA catheter (p less than .001). In contrast, for patients in cardiogenic shock the in-hospital CFR was 74.4 percent for those receiving a PA catheter as compared to 79.1 percent for patients in shock not receiving a catheter. The use of a PA catheter was associated with an increased length of hospital stay irrespective of the development of acute clinical complications. Long-term prognosis for discharged hospital survivors who had a complicated MI, for up to a five-year follow-up period was similar whether the patient did or did not receive a PA catheter during the acute period of hospitalization. In conclusion, we could not demonstrate a beneficial effect associated with the use of the PA catheter on selected patient outcomes, including in-hospital and long-term prognosis and average hospital stay, in this community-wide study of patients hospitalized with acute MI.