A bone marrow transplant (BMT) is a treatment for children with certain types of cancer or other diseases. The goal of BMT is to replace a child’s diseased bone marrow with healthy bone marrow. Bone marrow is the spongy material in the center of the bones that makes all types of blood cells, including young blood cells called stem cells. In BMT, stem cells are filtered, and then put back into the same child. Or they are put into another sick child. These stem cells will grow and become new and healthy bone marrow.