The practice of giving one’s stem cells through an allogeneic transplant to someone else for the treatment of diseases such as cancer or other immune diseases. In order for stem cell transplant to work, both the donor and recipient must match with similar ethnic background and the same type of immune cells. There are a number of stem cell databases or registries around the world where donors and recipients can register so that doctors can easily identify matched pairs.

Donors are required to donate stem cells by extracting bone marrow, a more complex procedure that uses a needle to withdraw cells in the marrow directly from a donor's pelvic bone. Another method is to extracts cells using a blood transfusion. The second procedure usually requires the donor to receive injections into his or her blood to "activate" the stem cells. Next, a transfusion machine extracts the healthy stem cells from the donor's blood system. Stem cell donors do not experience any negative health effects from the procedure.

People usually volunteer to donate stem cells for either because they have a loved one or friend who needs a match or because they want to help people. Some people give their stem cells so they can get them back later for an autologous transplant.