It has been known for more than 20 years that in some patients after revascularization partial or complete improvement of previous left ventricular dysfunction occurs. The condition for this improvement is viable myocardium, i.e. contractile reserve. Chronic reversible contractile dysfunction, also called hibernating myocardium heart muscle is along with the presence of acute reversible dysfunction--a stunned myocardium the prerequisite for this improvement. In the submitted review the authors try to present at least some contemporary views on the pathophysiological mechanism of the development of a hibernated myocardium. The normal blood flow through the heart muscle at rest in the majority of reversibly dysfunctional segments is a change as compared with the formerly accepted definition. The authors mention histological changes of the hibernating myocardium, in particular so-called dedifferentiation. Attention is paid to a review and comparison of methods used at present to detect a viable myocardium. From the aspect of clinical practice it appears that unless revascularization occurs, the presence of a viable heart muscle is a factor which causes a poorer prognosis of the patients.