New results with gene therapy for severe combined immunodeficiency due to adenosine deaminase deficiency are promising. We review a clinical project in which ten children were treated with gene modified autologous haematopoietic stem cells. After treatment, eight patients were able to do without enzyme-replacement therapy, and nine patients showed improved immune function and sustained low concentration of toxic metabolites. No clonal outgrow was observed indicating a limited risk for future malignant development. Despite these promising results, the safety of gene therapy can still be improved.