Severe combined immune deficiency disease due to a deficiency of the enzyme adenosine deaminase is a rare disease. However, it has been used as a prototype disease for the development of a variety of treatment modalities that are currently applied in more frequent diseases. For example, allogeneic bone marrow transplantation and stem cell gene therapy have been used for adenosine deaminase deficiency before being applied in other more frequent diseases. In the present paper, the development of bone marrow transplantation and stem cell gene therapy, as well as treatment with purified enzyme, for adenosine deaminase deficiency are discussed.