To determine the effects of early ambulation on peripheral venous thrombosis in the coronary care unit, 29 patients with acute myocardial infarction had daily 125I-fibrinogen point counting of both legs using a standard portable technique in the first 3 to 7 days after admission. Twenty-one patients underwent early ambulation during the initial 3 days, while 8 remained at complete bed rest for 5 days. Only 2 of 21 early ambulated patients had positive fibrinogen point counts, in contrast to 5 of 8 nonambulated patients (P less than 0.01). With heart failure, only 2 of 9 ambulated patients had positive point counts, compared with 4 of 5 nonambulated patients (P less than 0.05). In 16 patients undergoing venography, point counts were confirmed in 6 positive and 10 negative findings. These results show that the high frequency of peripheral venous thrombosis in immobilized acute myocardial infarction patients, particularly those with heart failure, can be effectively reduced by early ambulation.