In animals, brief episodes of ischemia before a total coronary occlusion protect the heart and result in a smaller myocardial infarct size. In humans, episodes of angina before acute myocardial infarction might also confer a preconditioning or protective effect; numerous studies show that preinfarction angina is associated with smaller infarcts. Preinfarction angina is also associated with reductions in ventricular dysfunction, arrhythmias and incidence of no-reflow phenomena, and, in some cases, improved survival. The protective effect of preconditioning in humans is characterized by marked individual variations and seems to be attenuated in women, people with diabetes and the elderly. Exercise seems to be an important way to induce preconditioning in humans and preserves it in the elderly.