How often does a bone marrow transplant not help with leukemia?

It depends.: The risk of relapse depends on a number of factors, such as whether the patient is in remission, the type of leukemia, and pre-transplant prognostic factors. It also depends upon whether the transplant is autologous (from themselves as donor) or allogeneic (from someone else). The relapse risk can be from modest to almost certain depending on these factors.

It varies: The specific subtype of leukemia, remission status at start of transplant, type of donor, type of chemotherapy or radiation used for the transplant are just a few things that are needed to help assess the outcomes... The long term survivals are 20-75% depending on these and other factors.