Today, there is much evidence suggesting that organ-specific stem cells need not rely completely on their own sources for maintenance and regeneration of an organism. In certain circumstances, mostly related to tissue damage, stem cell populations residing past the affected organ can contribute to its recovery--that means from different cell lines and also in tissues from another germ layer. The key factor in formation of self-renewing cellular clones is the presence of stem cells either from the tissue of origin or stem cells migrating from other areas and their successful settlement in an empty niche of the damaged tissue. Stem cell plasticity is the ability of adult tissue-specific stem cells to switch to new identities. The term plasticity also means stem cell phenotypic potential, which is broader than phenotypes of differentiated cells in their original tissues. Many laboratories have given evidence on stem cell plasticity; however, the presented results met with many objections from others. In the first part of our report we wish to refer to several issues associated with stem cell plasticity, transdifferentiation and fusion. Recent experimental results show that stem cells will play a key role in cell therapy. But there are still many questions to answer for scientists engaged in stem cell research. Is it possible to induce cells from one type of tissue to look and act as cells of another tissue? Do these changes occur naturally? Could plasticity be used in the treatment of fatal diseases? Cell therapy is one of the methods to treat damaged myocardial tissue. However, recent results with autologous bone marrow cells in the treatment of damaged myocardium show that this method has still many unanswered questions concerning cells, cytokines, microenvironment and other factors responsible for reparation. To date, there are many opinions either recommending or denying this method in different modifications. One question has not yet been definitely solved: What are the conditions for us to accept this method--its safety and efficacy? The future will show whether these our hopes and expectations will be fulfiled. Many experiments are needed before at least some of these questions may be answered and cell therapy become an important method for the benefit of our patients.