Men were randomly assigned to regular aerobic exercise or habitual activity. Aerobic exercise included walking, jogging, cross-country skiing, swimming, or cycling. Target exercise times were 45- to 60-minute sessions 5 times weekly. Men in the exercise group received heart rate monitors and kept an exercise diary. Men in the other group were not told to change their exercise patterns, and they did not keep diaries. Men were followed for 6 years. Each year, they took a bicycle exercise test to measure breathing fitness. They also had ultrasonography to measure the thickness of the walls of the arteries in the neck (carotid arteries). Experts who read these tests did not know which men had been exercising regularly. The researchers then compared groups to see whether those assigned to exercise had less thickening of the artery walls (atherosclerosis).