Ischemic preconditioning has been shown to be one of the most powerful means of protecting the myocardium from ischemic injury in experimental animal models, although the mechanism is incompletely understood. In this review we discuss the evidence for preconditioning occurring in ischemic syndromes in humans, whether the human myocardium can be preconditioned, and whether preconditioning would have a place as a therapeutic tool in clinical practice. Some studies evaluating patients after acute myocardial infarction have shown a better outcome in patients reporting angina before the onset of the infarction, but this is not a universal finding, and it is difficult to exclude other confounding factors, such as collateral flow, from influencing the results. More controlled prospective studies have evaluated patients undergoing percutaneous transluminal coronary angioplasty and have found less ST-segment change and less reported angina during the second balloon inflation when compared with the first. Again, it is impossible to completely exclude other causes for this effect, but the dependence on mechanisms that are known to be important for preconditioning in animal models does suggest the phenomena are the same. Further experiments using isolated human atrial muscle have shown that human myocardium can be preconditioned and that the mechanisms involved are similar to those elucidated in animal models (adenosine, protein kinase C, and ATP-dependent potassium channels). In clinical medicine preconditioning is most likely to benefit patients when it is used to protect against the ischemia induced by cardiac surgery. In this respect, a study has shown that in patients undergoing coronary artery bypass grafts, the reduction in ATP occurring during the first ischemic period is attenuated in those given an ischemic preconditioning protocol beforehand. Despite these advances, it is likely that the full potential of preconditioning in clinical practice will not be realized until the whole mechanism of protection is understood and a safe pharmacological "preconditioning" agent becomes available.