A consecutive series of 174 patients with acute myocardial infarction (MI) was prospectively monitored for pericardial friction rub to diagnose pericarditis (PER) and had three echocardiograms on days 1, 3 and 10 of hospitalization, to detect a pericardial effusion (PE). The relationship of heparin therapy to PER and PE was also studied. Patients with PER (n = 41, 23.6% of total) had a higher rate of anterior MI (65.9 vs. 36.8%, p < 0.001), heparin therapy (100 vs. 84.2%, p < 0.001) and PE (34.1 vs. 15.0%, p < 0.01) than those without PER; also the PE on day 1 was smaller in the patients with than without PER (0.5 +/- 1.2 vs. 2.3 +/- 2.7 mm, p < 0.005), but not different on days 3 and 10. Patients with PE (n = 34, 19.5% of total) differed from those without PE only in the incidence of PER (41 vs. 19%, p < 0.01). PE increased between days 1 and 3 (1.5 +/- 2.3 vs. 3.7 +/- 5.3 mm, p < 0.05), but was not different between days 3 and 10, during which it was 3.8 +/- 3.3 mm. No higher rate of complications was noted in connection with PER or PE, either in the hospital or for a mean of 20 months after discharge. Heparin therapy in patients with PER and PE did not have an untoward effect. Hence, PER and PE were frequent in patients with MI; their emergence even in patients receiving heparin did not alter the expected clinical outcome, both in the hospital and during a mean 20-month follow-up after discharge.